I really like M90s. My M90 has the original L-Jet and while the parts are much much more common than the D-Jet parts, it's getting almost impossible to find fuel injection relays, throttle position sensors and some of the ignition parts. There are people making some pieces but not for BMW applications generally. An M90 with a more modern fuel injection like that of a Motronic 1.3 setup from an M30B35 is fantastic. Parameters of the motor are similar and it works well, uses pretty commonly available parts that are often never more than a day away from anywhere. Of course, for how long is anyone's guess since I used to be able to get an L-Jet combo relay same day and that was only 5 years ago. Either way, you can easily future-proof this with a good cache of the important spares.
That said, you'll spend a bunch more building an M90 for minimal gain. Similar power or better with a bump in compression can be achieved with an M30B35. The head flows so much more than the M90 head (the M90 head is what became the M30B34 head, but it was introduced for 1980 in the M90. The M90 casting was also used on M30B32 in the US for 1983). The early M90s used the tiny port crack-prone M30 heads, so if you buy one of those motors you're basically buying the bottom end (78-79 build M90s). Those blocks have more webbing and gusseting on the outside than the 1980 blocks. Then there are Motronic M90s, which are different still, have the 93.4mm bore and the 84mm crank, but from the outside look like an M30B34 because of the truncated water jacket on the left side. None of this is important since you should get an M30B35. It's more than plenty of motor and I used to tell people just about anyone can work on one but judging by a car in my shop right now, that might not be the case.
Of course, I'd buy a car first and then figure out what to do from there. Buy the best car you can afford even if you have to stretch the budget a bit. You could save yourself £50k. Better yet, see if you can drive a few coupes with different motors and see what you like. You might be surprised how the car feels when you're not thinking about the numbers.
