The four-cylinder E36s are slow, but don't have the same severity of cooling system issues that the sixes do.
Aside from the early profile gasket failures, the M42 cooling systems are bulletproof.
The switch to some plastic components (thermostat housing and a pipe on the rear of the head) on the M44 mean they can eventually fail, but both are external components that aren't that difficult to replace.
But neither the M42/44 have issues with their water pumps, or radiators/expansion tanks that the sixes do. Since they also have remote coil packs and lack the coil-on-plug ignition, coil failure is also not an issue.
And while the sixes are no slouches, something often overlooked is that fours (of both generations) are just so ever slightly sweeter handlers because of less weight over the front axle. (A GM LS V8 weighs less than the BMW sixes--that transplant is also a fun project).
BMW tried a lot of new things with the E36, and eventually patched up many of the issues. While the design is good, interior build quality was substandard to begin with, and only managed to reach mediocrity, though things do look nicer on the later cars. The electronic HVAC controls are more potentially troublesome, though.
Stiff suspensions, poor roads, and/or abuse can cause the rear subframes to tear from the body (something that also affected the E36 7/8 Z3s and continued with the E46s!), but it's not a chronic issue by any means. M3s had the reinforcement plates installed at the factory, and though labor intensive, they're something a hobbyist who can weld can install as well (same with the rear sway bar tab reinforcements; again, something normal cars are unlikely to need).
The E36/5 compact is bit of an odd bird. Some people like that the trailing arm rear suspension makes it more lively than the more buttoned-down Z-axle. But overall, it's not quite as refined in terms of NVH as the other E36s. The interior fittings and HVAC system were also designed with lower cost targets; not a huge difference, but something to note. The long doors (longest of any E36) sometimes manifest themselves in the window hardware failing and causing the glass to derail (but the regulators are still more solid than the Mickey Mouse cable design in the E46s).
If you like the E30, the compact is the closest you'll come to one, in E36 clothes.
To me, the E30 is the last of the old-school BMWs. The doors shut and latch with a raw metallic "snap" like older German cars do. The interior is very clearly driver-oriented. In 318iS form, it's light, very sweet handling, and the last evolution of the E30.
It will probably come down to preference, and finding a good example.
Even an E46 would be ok, before BMW started to lose the plot with the E90 and later.
The company is now ruled by the bean counters (an opinion shared by execs who have departed), and while it's undeniably doing well as a business, the cars don't have the same appeal for enthusiasts.
