I being a car builder will tell you what I tell all my clients, its better to build what you want. With that being said most of us have a budget when building a car so your best bet it to buy the most you can for your money and make sure when you are looking you either now what you are looking for or take someone along that will help you make a good decision. Then take that car and make the changes you can to it to make it your own, half the fun in owning these cars is the building process.
The nice thing for us is that we get to build alot of different cars and we help clients find them or they bring what they have found to us, in most cases they were better off building it from a junker then buying somethinig that was already built because of all the stuff that people hide out there today.
10 years ago you could buy most muscle cars from honest people just trying to sell a car. Today you have hundreds of people trying to make a quick buck on a junker that they have done a lot of shady work to. For example we had a client bring in a 70 Chevelle SS that he paid $30K for and it looked great but when we started digging we found lots of rust so we sent the car out to be stripped and when it came back we had to replace everything on the car look for yourself
http://www.feslerbuilt.com/1970Chevelleshame.htm
Just make sure you take your time when looking to spend your money on a classic and if you know what you are looking for you can save yourself a ton of money and time on the build. There is nothing worse then having to start a project you think is almost done over. Buy a car that has all the body work done but no paint so you can see what you are getting you can find projects all day long that someone can not afford to finish and just needs to get out.
If you go on EBAY ask for a ton of pictures of the underside and inside everything, under carpet, inside the trunk, complete underside of car and if there is a ton of undercoating what are they trying to hide, if they wont take the pictures or tell you they cant there is something wrong with the car move on to another one.
If it sounds to good to be true it usually is, so again take your time and get a friend with some knowlege or pay someone to look the car over for you, get it up on a lift and check everything out. It will save you a ton in the long run. I hope this helps you a little