Thing's you'll need to check are:
-Documentation: Year of manufacture, Complete file, Token's paid upto date, Return file, Sale's recipt from previous owner( if their was any), Original number plate's, Original book.
-Exterior: Check the entire body for any Re-painted/Repaired Part's, Check the Chaissi/Frame for any sort of damage, Check the vehicle's under-body for any rust/welded part's & Piece's.
-Engine & Suspension: Get both, throughly checked from a trusted mechanic as engines vary from car to car.
-Interior: Check for any type of welding/jointing on the flooring, check roofliner, check all pillar's (inside & outside).