390's are very easy to find parts for. Plus there are lots of upgrades available including aluminum heads and intake. As Ray suggested take the checklist along with you. There are trouble spots to look out for particularly the floors and inner and outer rocker panels. Also check the top of the shock towers as well as the battery tray and sheet metal. Pictures can hide a lot of poor repairs that can wind up costing more than the car is worth so it pays to do a thorough inspection. I wouldn't be so concerned with the mechanical condition unless the trans has a major leak or the engine is frozen or knocking. Most of the parts are pretty inexpensive. If the car hasn't been driven regularly you can count on doing a complete rebuild of the fuel and brake systems.