If you are taking the plunge into the hobby and have reached the stage where you are getting ready to make your purchase, there are a lot of things you should know before you blow the wad.
Once you have figured out what kind of car you want to buy, you are going to have to do a little research. It is always a good idea to determine what a car's weak points are, whether those weak points are original design flaws or regular abuses that some cars such as Corvettes are subject to by their owners, i.e.. continuous Jack rabbiting, excessive braking etc. In most cases a seller is not about to offer up a list of things that are wrong with the car unless he is trying to cover up a large flaw by admitting to a few small ones. This leaves the responsibility for determining the car's fitness to you. If you know nothing about mechanics and body, you can get counsel from a professional in the field like a mechanic or dealer, but beware the well intentioned friend. I have seen situations where friends' advice to prospective buyers has implemented their purchase of a very bad car and indeed the reverse, where a really good car has been passed over because the friend was more interested in showing off just how much expertise they had rather than being objective about the car. You also have to be aware that not everybody who works with cars has the expertise to help you select a classic. Joe down the road who services your Honda may be just great, but what does he actually know about a 1959 Oldsmobile or a 1934 Buick. In that situation he may be just as lost in the wilderness as you. He may never have even seen under the hood of one of these cars, never mind knowing about carrier bearings on the drive line, torque tubes, or cast exhaust manifolds that have expansion joints that are prone to seizing and cracking. You may end up putting Joe in a very bad position, especially if you are paying him to make the call. If he calls it wrong, and it costs you a ton of money, you might lose a guy who's great at fixing your Honda, and he might lose a good and valued customer.
If a seller is willing and you have a reputable antique car dealer or restoration shop near you, it is best to take the car to where it can be properly inspected. If that is not possible, then you are going to have to rely on yourself and your own common sense. Never let your excitement about a car blind you to its warts. Always remember that there are lots of other cars out there.
The first thing to look at is the body and paint. Walk around the car and look at all of the door, trunk and hood fits. Let your eyes track down each crack, making sure it is even and that there is no pinching or opening of the gaps. They should also be even all around the car. The gaps on either side of the hood or on the left and right front doors should be the same. If this is not the case then they could be telling you that the car may have been in an accident which has been inexpertly repaired, or that it has been through a poor restoration, and might be an indicator of many more problems under the surface. Any competent restorer knows how to fit the hood and doors. If they are wrong then it can show either an incompetent restoration or worse, that there is a serious problem with the car that can not be corrected with adjustments.
Next, check the rocker panels and inside the fender wells and inside the doors. These are areas prone to rust and often glossed over in a quick fix by just stuffing with bondo and covering with gravel guard. If you find that the lower rockers are covered in gravel guard or wavy and apparently stuffed then it may be a good idea to look no further and take a pass on the car right at that point. The only thing worse than rust is badly repaired rust damage. It is usually harder to fix and as a result costs a lot more money.
Open the doors and see if they drop a little as you open them. Lift up and down gently and see if there are hinge problems. If they are rigid, that's good, if there is a lot of movement, take a good look at the area around the hinges and inside the back of the front fender as revealed by the open door. If it doesn't look right, it probably isn't.
The door skin wraps just over the edge of the door and is crimped and spot welded on most cars. This edge should be crisp and straight.
If it isn't there or disappears under bondo, it shows a bad repair and also indicates the level of incompetence of the restorer or body man. Check the door sills as well. A lot of mudslingers (another name for a bad body man) are too lazy to sand and finish these areas properly and bondo may be very apparent. The same can be said for the rockers. A lot of poor body men can't be bothered to get far enough down to finish these areas properly. The same can be said of bad painters not wanting to bend down far enough to get paint under the rockers. If you get down and take a look under you might see unsanded bondo, unpainted surfaces and even patch panels with rivets in them. It's really worth the bend.
You should also take a good look at the paint. If it appears that the car has just been painted be suspicious. Fresh paint can often hide problems that will literally surface within months or sometimes even weeks of your purchase. Paint will often hide temporarily rust which has just been painted over. Be warned. You should also check to see if the colour is even all around the car. Patches of colour which are a little different may indicate blow ins, which are localized repairs in which the entire car is not painted but just damaged areas. There is nothing wrong with a good blow in, but if it is a good one, you probably won't see it. Bad ones can, once again, indicate a quick repair which has just been done to sell a damaged car.
Paint quality can be judged by the shine and the amount of orange peel in the paint surface. Orange peel is a condition that exists when paint has either been shot with too much air pressure and lays on to dry or has too much solvent in it which causes it too dry too rapidly. It actually resembles the surface texture of an orange, and if it is really bad, it ruins the appearance of the car. Believe it or not, new cars are often covered in orange peel because modern production techniques demand that paints dry very fast, so production paints often contain accelerators which cause peel. In order to match this appearance many modern paint codes actually replicate this look and if a 1996 Chevy is the subject of the paint job, that's fine, but bad or inconsistent orange peel looks bad on a classic. If the paint on the car is good, it should show a crisp light return. If it is fuzzy or dull it could be orange peeled.
Another little visual test is to sight down the sides of the car examining the reflections. Bad bodywork under the paint is often revealed by reflections that are distorted.
Once you have had a good look at the body and paintwork, and if you are still interested in the car, it's time to get down and get a good look under the car. For this you really should have a flashlight. Look first at the frame and body mounts. The body mounts are usually on arms that stick away from the frame. Check to see if the rubber mounts are in good shape and the supports are not rotted. On most frames, some rust, especially surface rust, is the norm, but you should, using a screwdriver or similar tool, just tap suspicious areas on the frame with the point. Soft areas are easily detected doing this. Look at the bottom of the floorboards and the rest of the passenger compartment including the base of the firewall and the areas around the gas tank at the back of the car. You should also look in the trunk at this time and check under the carpets and side covers. Stick your hand down into the well on each side of the trunk and feel around for rust or other problems. The flashlight may come in handy here as well.
If when you are looking under the car all you can see is layers of tar guard and spray on under coat on everything, it is time to reconsider how much you want the car. It is amazing how much damage can be concealed by this stuff. You should feel happiest about the bottom if what you are looking at is old, even worn factory under paint and frame paint with perhaps some surface rust. Don't let yourself be fooled by spray on tar.
Checking the paint and body work, aside from a few tips, is common sense. The further the car appears to be from correct, the less desirable it should be to you. A few faults may be all right. It depends on what you want and how fussy you want to be, but if there are a lot of things wrong, unless you have deep pockets or are a masochist, take a pass.