Hi elclassico,
The top concern is rust. Early 80's and earlier BMW factory antirust is not good, I think the body is not fully galvanized. I think early E30 is also affected, but not as
bad as E21. My uncle's ex E30, the new owner is preparing for firewall rust repairs. The car is the prefacelift model.
Unlike E30, halfcut of E21 are very rare.
Best E21 to restore is the 323i. It's the top of the range model.