There are many ways. You can probably fit almost any strut into the e21 with some modification (which could be major or minor). Ideally, find yourself an abandoned e21 3231, take the front struts and the rear trailing arms and disc brake assembly, refurbish and install.
You may have to replace the discs (RM120 - RM130 each, front and rear). Check that the caliper pistons are in good condition (i.e., not corroded/seized) and have them serviced. Alternatively, use Girling 4pot calipers from Volvo 240 (RM1000 a pair - brand new; al lot cheaper - used) Requires some modification.
Change your brake booster to the one from the e28 - minor knocking on tyre well and cutting and welding booster mounting.
Suspension - I like H&R. You could do a group buy with -berq-
People who have been in my car can tell you how my ride feels.