Oh dear god. The list is quite long mate.
Rust:
Rear arches
I would open the rear side windows and check under where each end of the rubber seal joins as that can harbour rust
Have a feel under the scuttle panel as well, underneath the lower edge of the windscreen (going via the plastic cowling openings) for rust. Again the seal at the base of the windscreen is poor and sludge can build up over time and I suspect there are lots out there with potential problems in this area that they don't know of.
Tailgate drainage holes rust as well
Mechanically:
Check CV gaitors, blue smoke, mayo in oil cap, general listen at the engine for tappety-ness and how it idles. Idling problems can be a ballache to sort.
Check shockers for leaks and check the struts for play which can be the top mounts going. Check for play in the steering with the steering lock off but no engine. Steering column bushes are a common failure. Check the gearbox for it popping out of gear, and also check the clutch. They are quite heavy anyway but should not be excessively so. If they are either the cable is fecked, the clutch is going or (as inmy case) the wrong cable was fitted.
Check brakes obviously and all fluid levels. Keep an eye on the coolant temps when it's up to temp. Should sit at the 2nd mark 2/3rds of the way up the gauge. Oil temp should be 90-110 and pressure should be about 3 bar at cold idle falling when warm to between 0-1. Should rise healthily when moving to 5-6 when cold or 3-4 when warm.
Then beyond that it's the usual checks really.
Good luck.