Where do I start?
Rust should be your main concern, 'cause it can be big dollars to get it cut out-
*Check out the sills (panel underneath the doors)
*Also check at the base of the A pillars (underneath the windscreen) by opening the front doors and feeling in the gap there.
*The rust on the door pillars? you describe is a worry, if its bad around the hinges you'll have to get that fixed
*Also check the usual spots in the corners of the doors, lower front and rear quarter panels.
Have a good look around for overspray to see if there has been any dodgy repairs done. Use a fridge magnet if you suspect there is any bogged up rust (it wont stick to bog!)
I think that's about it for rust, someone else might think of somewhere else to check
I suspect either the gearbox or engine is not original, AFAIK
all ZH 302's had a C4 g/box and all 351's had an FMX, could be wrong though. Not that it really matters. Just do a compression test to see if the bottom end is OK.
Heads would also need a recon after 280000k's if they havent already been done
Hard to put a price on it without seeing the car, but if it's out of rego and needs new rotors and pads, I wouldn't pay anymore than $1000-$1500 if it's in average overall condition