[SYT_Shadow]

I would definitely do rod bearings on the used engine before putting it in.

Let me explain what I'm saying overall differently: if you don't do your own work, buying an 85k mile M3 is a HORRIBLE idea. I don't know what performance car can be bought used at 85k miles and taken to 150k miles for much less money, but certainly not an M3! (and in my opinion, no performance car on the planet)

With those costs you quote of the LSs, I have no idea what anyone is doing in an M3. Five years back, I paid $5k to have an engine pulled out of the car and then put back in. So if you can buy an LS engine and have the old one removed and the new one put in for 2-4k, definitely stay in that platform, because it sounds like someone is paying you between 1 and 3k to take their LS engine.

FRMs are common issues.
Valve cover gaskets are maintenance, and they fail in every single car sooner or later. 115k miles sounds pretty damn good to me!
You had one weird issue which is the axle hop topic.

There are some very long threads about predetonation on certain tunes, and that's all I'll say about that.