yep no point bothering with M20 unless its for a specific racing class, you just want to keep the car "more original", or you want the cheapest option to get the car back on the road.
M30 is dinosaur tech and boat anchor. was the engine of choice for decades when M5x engines were over 2/3k, but no point doing that now.
i have done a bunch of M52 swaps, its easy to do and my engine of choice. just do plenty of reading, and read through a half dozen build logs and take the average of suggestions as the fact, dont rely on any one person.
Bear in mind two things, 1/ most of the logs will be in American which means they are left hand drive (important when it comes to exhaust stuff, steering, and clutch control), the pinout tables are not correct for your car, and also 2/ most of the threads if not all are outside of NZ, where rules are generally a lot laxer and do not require the certification process we have. thus you will need to think about things like drive shaft hoops and bump steer tests, possible be forced to upgrade brakes etc. worth looking through the LVVTA Threshold guide as a starting point.
also i recommend swapping to stock E36 steering rack at the same time, they are cheaper now than they used to be, and cheaper than having to make up custom power steering lines to adapt the e36 pump to the inferior E30 unit. use the holden barina / opel corsa linkage youll see mentioned, e30 nuckle on top and e36 on bottom. also means you dont need to recert later down the line when you do it anyway.
having said all that, if i do another swap in the future, i think i will use the M54B30.