Buying a classic junk car can be for several reasons such as wanting a project to work on, or having a restoration project that needs several parts or bodywork replacing.
You will still need to follow standard protocol when buying any used car, making sure that the title and VIN are all ok and the seller does have the vehicle to sell.
If you are wanting it to restore, you will need to be realistic about the condition of the car, and how much will it cost to get it to the stage you wish. This can become emotional if you have always dreamed of having a particular car, but such dreams can quickly tour sour if your costs escalate beyond your intentions. If you are intending for a garage to do most of the restoration for you, take the car to them for an honest evaluation, they should also be able to give an estimate for you finished product. If the car is not currently running, find out why. It is one serious problem, or a series of smaller problems that will take time and money to investigate and fix. Check out the state of any bodywork, look for rust and if this has weakened the integrity of the vehicle. Certain cars last longer on warmer climates (less grit on the roads when it is winter, so less corrosion) so ask about the car’s history, where has it been most of its life, and how hard a life has it had (look for mileage)
If you are looking for a classic junk car merely for parts, put together a list of parts that you need to work. Make sure any purchases you make include these parts and do your best to test them. This can be difficult if the engine is not currently running so you will have to ask the owner if these parts worked previously. Unfortunately this may have to be done on trust, so ask as many questions as you can and if you are not happy with the responses or answers, you do not have to go ahead with the transaction. You can also lower the price you offer, for the contingency of these parts not working. Then you may not have missed out on too much if they do turn out not to work.
It can be a tricky world to dip your toe into especially if others around you know more about cars and engines. Do not be afraid to ask for advice from those you trust, and if you know anyone who has restored a car, they may be a great to come shopping with you if possible. Pay attention in any owner forums for likely flaws and break downs, this means you can pre-empt any failures and know what parts to keep as standard spares. You are likely to face some pitfalls, but as you continue you will quickly gain knowledge that will become invaluable to you in future years with your classic car.