Great thread this, love that e12 as well, mmmmm yes! The P38 Looks much better on the 235's to boot.
Not much to add on the rangey front but I ran an M51 in an e34 for a few years (mostly on veg), didn't get on well with a chip (Not much appreciable difference or just too smokey), Map is a preferable choice but the amount of parameters you can actually change on the old leccy VE pumps is minimal - they're still mostly mechanically governed internally. So in terms of "remapping":
Two best (i.e cheapest!) mods I found, were to advance the pump manually a degree or two - achieved by rotating the entire pump body, by loosening the 2 FIP mount nuts just enough to turn
(No. 1 in photo) - the studs have a good 10mm or so of adjustment space, to help offset for engine wear. (clockwise rotation of the pump body facing the front of the engine, i.e moving the pump toward the engine block = advance), 1mm increments of movement is a good start, usual rules of adding advance apply. This also will largely fix a hot start issue without a chip / bodge as a bonus, and will go some ways to fixing a wonky / wobbly or unstable idle / wobbly return to idle, owing to wear on the throttle potentiometer strip in the pump.
The second is to add a resistor into the earth line on the fuel rack position sensor of the pump, think the circuit is pin 2 and 3 on the plug (
No. 2 in photo, can also add anywhere pre-ecu in the loom) (on the tds anyway if memory serves) - you can essentially then "bleed" off the signal reaching the ecu, giving you a vast amount of potential to adjust injection quantity/duration. (best bet is wire a 1k potentiometer in instead of a fixed resistor while you tinker to find what it likes). The TD04 turbo is quite well suited to stock output and can manage a very small boost increase without dropping off the thermal efficiency scale as well. Most folks don't put much confidence in everything behind the engine however, so mod with care
You can also shim the dynamic advance pin out
(No. 3 in photo). Never actually did this on mine - search boost ring for explanation
Rebuilt injectors also make a world of difference for running quality, MPG and power, if you're running original ones with over 100k~ on them
Maintenance wise after 180k or earlier if it has a shady service history keep an ear out for the cam chain starting to rattle - one of the few major problems that can develop, and if you ever take the head off, have all the prechamber cups replaced, they're cheap from bmw and any decent machinist will remove, clean the face, then knock new ones in to the correct depth for less than 50 squidders.