I have bought a few light aircraft and here is my humble opinion on the matter.
1. Do your homework on the costs, (the real costs, not wishful thinking)
2. Deciding on what model and category can be a tough decision, define what the mission for the aircraft will be now, and just as importantly, what the mission/use will be a few years down the road i.e. purely for touring or advanced training etc. Do not buy a plane just because you "like the style/design" of it. mission profile is fundamental.
3. Valuing. is an art, due to the numerous variables i.e. seller motivation, model popularity and availability and your negotiating skills. However you can get a guide by looking at the prices listed on places like trade-a-plane or the controller for comparison.
4. Reputable mechanics are relatively easy to source for a pre-buy inspection, however, firstly, do your own homework once you have decided on the model if its a category A production aircraft go on the FAA or airframe/engine/prop manufactures website and download all service bulletins, then you AND the mechanic go thru the airframe and engine logs and read them from start to finish and verify all routine and mandatory service has been performed and by whom. and look for things that just don't look right in the chronological entries.
5. Try to find out about the history of the aircraft that will not be in the log books such as, has it really been hangared? look thru the old maintenance invoices and see where it used to be based and visit the airfield and ask around about the plane, low airframe or engine time over a long period can be as negative as high times.
Good luck