Although these cars are legends and will appreciate, In mint shape, it's worth $10k tops, in average to good app. $5k.
Here's what to watch for: a. Timing belt; have it checked asap and replaced, its worth it b. Shock mounts; if you hear creaking noise from the rear. c. The plastic window of the top; always place a cloth where the window folds d. Seals; tranny, engine and headgasket; they dryout with time - there is also a seal called the 'guibo' which is known to go e Electrical; ultinator, starter, waterpump; have them checked
Tips: BMW dealers do a cheap '55 point' inspection, it's a great way to find out what you need to fix and work away at it.
Here's what will make it worth more: a. Hardtop: original black hardtop, with grey headliner b. Complete toolkit under the trunklid c. Original manuals and stickers inside the door sills d. Front foglights, 13 button UBC, cruise and 'sports' shift option e. Original/replacement floor mats with BMW f. Original spare tire: Michleyn MXV 195 RV g. Original black cassete player