i worked in a BMW dealership for 18 months. Not the most reliable cars, and have expensive parts too. The Z3`s you mentioned are sensitive about their radiators and thermostats. A mate also has one, also had differential issues, gearbox replaced and says the leather seats are shite an can confirm that from my own experience. And most common in all bmw`s are oil leaks around the block and/or head.
oook news to me
been working in bmw dealerships as a bmw tech for nearly 6 years now
kinda depends on what engine the car your looking at is. if it's the M43 single cam 16v 4 pot engine their pretty terrible. oil leaks galore by now thanks to some rather fiddly timing case gaskets. but their loads of room to work on them and their not very expensive/hard to do pretty much avoid this engine like the plague. while they can be reliable their generally more trouble than their worth
the 6 cylinders are spot on. just make sure theres good service history. any shrieking or grating noises from the front of the engine especially on cold start is liable to be one of two things. the waterpump or the aux. belt tensioner
either way your best replacing the water pump aux. belt and tensioner. you can't get the mechanical tensioner for them anymore as they where changed to hydraulic tensioners. so you can buy a conversion kit which isn't too expensive.
brake pipes are well worth checking as are fuel pipes as their all quite exposed they seem to suffer from the uk's poor weather/salt roads
rear diffs are solid in all models so long as they aren't leaking/allowed to run dry. just make sure to check the rear diff hanger bush as this can split and cause a banging noise. to replace you can buy the rear casing of the diff complete with gasket and bush for a reasonable price
any sloppiness from the steering is liable to be down to front lower arm bushes. which are cheap and easy to replace
check the front anti roll bar mounting baseplates (the ones welded to the body) as some heavily corroded cars have had the welds snap
servicing ever 2 years or 15000 miles
oil service - oil, oil filter, microfilter
inspection 1 - oil service + check fault codes + mot style check
oil service
inspection 2 - as insp. 1 + spark plugs, air filter ( fuel filter is not changed)
theres also 2 other services that are due when the clock symbol illuminates on the dash when the ignition is first turned on
brake fluid change @ 2 years
coolant change @ 4 years
gearboxes are also very solid and shouldn't provide too many problems apart from the usual abused clutch issues that some owners cause
roofs are fairly reliable though the plastic rear window is prone to turning opaque then eventually cracking. this can be replaced seperately from the cover and most of the time is fairly straightforward but sometimes it can become a complete nightmare when the zip fails
the interior isn't great to be honest but i's fairly solid and reliable
if the windows don't move when opened it's liable to need some plastic slides replaced in the regulator (cheap and easy to replace)
hth