AN ABSOLUTE MUST...
you must get a thorough pre-buy inspection. I don't mean a quick walk around, look at the paint type.
You must pull panels and above all, have an IA check the status of A.D.'s. Also, you must get a complete title search and FAA download of all previously filed FAA Form 337's. I can't tell you how many owners bring me modified aircraft with no documentation. They all look pretty sad when I tell them that its going to be X amount of hours ($$) to research, document and submit a 337 for something they were previously flying around with.
For example: a simple annual on a 172 revealed 7 major repairs and alterations that were not documented,, plus I discovered it had the wrong prop on it.
So, please, do yourself a favor, get a thorough pre-buy inspection... there are too many good airplanes for sale out there to waste time and money on a piece of crap.