Thanks for all your replies. Think I will start by finding some older manuals and peruse them. I am used to reading industrial controls manuals that originated in German and were likely translated to Jinglish in an old version of google, so should be right in my wheelhouse.
I noticed most recommendations were towards 3 series. Is that just preference or is the 5 series something to avoid?
Traditionally, the 5ers have been executive sedans with a sporty character, but nicer and plusher, given their higher price points.
The 3ers were smaller and sportier, and the lower price point reflected in some of the components.
There will be a lot more 3ers on the market, but more of them will be modded, or clapped out because the owner(s) didn't, or couldn't afford to maintain them properly.
The 5ers will have attracted a more mature buyer, more likely to have the means to buy and maintain them, so it's easier to find them in better condtion. But even those owners can tire of the maintenace or repair costs, and desire to offload them, and/or simply want something newer. A car coming from such an owner, in an off-market sale, would be a gem.
In general, the less complexity there is, the less potential risk, or problem areas there will be. V8s and AWD increase those risks. More luxury gadgets.
The 2002 is considered the archetypical BMW. Small, light, and simple, with a 4-cyl with good power.
But that was a long time ago, and as the lineup grew, the sweet spot became a 3 or 5 with a silky-smooth inline six.
The big 7 didn't really become competitive with its intended targets, the S-classes and such, until the E32 generation, and then they also stuffed a V12 into them.
For a BMW cultist, the successor E38 7er is probably the pinnacle of that class. Large and comfortable, but no excessively so, but more importantly, still with the fun and soul. Those willing to tackle the issues with the timing case on their V8s could find them relatively cheap from owners who wanted to dump them and not fix them. It's a straightforward, but time consuming and thus somewhat costly job. Its successor, the E65 7, introduced the "Bangle Butt" (though it was actually styled by van Hooydonk) and awful first generation iDrive telematics to the mix. The formula was changing.
The newer models have strayed farther and father from those eariler ideals, even the M models, but that's a topic in itself.
It depends on what you're after.