The first thing to do is check its legal status. If it's on the road, is it correctly registered and has it passed its SVA/IVA test? You also want to see as many photos as you can of the build to make sure that it was done well. But you can also use the checks you would for any car - does the engine run and pull as it should, are the brakes even, do all the electrics work. Once you've narrowed down what kind of car you want, get in contact with the owners' club and see if they can help out with the car's history and paperwork checks. Have you considered building a kit car yourself? It's hard work and challenging but is also hugely rewarding.