where the rear beam attaches to the chassis is a common rust issue, and expensive to sort - easy spot by tapping the area it will appear soft.
heater matrix, if it hasn`t burst by now chances are its not going to, id say not to bother spending.... however a thing to check is that the heaters work on full for 10 mins of driving, if not its probably the bypass valves sticking (these are obsolete, but most just take them out - and risk a hot water scald on your feet) but as said if its not gone pop it prob wont now...
under the windscreen generally goes rotter around the bungs, as said.
front door pillars (bottom of A post) rots and gets holy.
sills go, but usually where they have sustained damage and arches generally go on the gtu as they are hidden under plastic and moisture is retained - the poverty spec seems to be better for arches in my experience.
gear linkages wear, but can be set correctly - no drama
doubt you will be looking at a jetta gti, so no need to worry about calipers - but watch out for corroded handbrake cables, my betting is most of the calipers that are changed on mk2s are down to rotten cables letting in water and this causing a binding.
areas around the front strut turrents, battery tray (and under air filter), rear strut turrets (may be hidden by carpet), rear panel/valance & boot floor are rot magnets also.
window regualtors seize so check these out, headlamps leak moisture, rear lamps leak rain, door handles give up with years of use.
carb models suffer carb icing even more so if the induction has been fiddled with, i.e filter.
front subframes rot
then you have the usual shockers leaking, suspension wear, wheel bearings and clutch issues related to an older car -
also as its a vw and has probably been on the scene you have other munters work to put right too, this is what i cringe - everything up there is minor but if you have had a tosser attempting to do work on something he does not understand then you will be chasing your arse for ages..