Quick update: The top-end went together without too much drama, and I've been driving the M3 for the last three weeks to make sure everything is okay. I kinda feel bad about scrapping a nice Estoril Blue M3 Sedan, and I probably would have kept it had it been a coupe with a manual transmission. I'll start advertising non-needed parts on Craigslist soon (PM me if you need anything).
Anyway, the engine checks out nicely!
I borrowed a friend's leak-down tester and bought a Harbor Freight coolant pressure testing setup. I had no more than 5% leak-down at 100psi shop air on a warm engine, and the cooling system held steady at 25psi for two hours on a cold engine. The engine pulls strong to redline (as often as I can!), and it idles perfectly.
I just purchased a Z3M/S54 Behr radiator, Stewart high-capacity water pump, and a slew of gaskets/hoses from Pelican Parts in preparation for the swap. I hope to begin the swap into my 318ti next weekend; my new deadline (the next autocross) is March 12. We'll see how my 332ti compares to my Z3 M Roadster (with 198k miles); stay tuned!