I dont own a E36 320I, but i`ve been around cars for a long time and had a E30 318is and currrently an M140i.
A 320i should have the M52B20 (6 cylinder), those are solid engines, very easy to work on. Contray to popular belief, old BMW drivetrains (Engine > Gearbox > Differencial > Axles) are quite easy to work on, specially because of the longitudinal layout. I dont know about parts availability in SL, but there are plenty of parts all over the world (and online).
The way I see it, interior quality/materials of the E36 is not as good as the previous E30 or the later E46. Most european makers were experimenting with interior materials and functions in late 90`s/early 2000`s, BMW also got some things wrong (i..e. poor qualiry door cards etc...). Body panels and mechanicals are solid, though some rust issues possibly evident in SL cars.
Like most older cars, one needs to learn and understand what`s involved in maintaining these, it`s all relatively simple.