I'm not normally one to question abilities, but if you're an experienced shade tree mechanic, then you'd know that a failing M30 head gasket is hardly a reason to replace the engine. In fact, it can be done in a few hours without much work at all (even without removing the head or intake from the car). M30s die from abuse, not wear. A little top end work is expected around 200k.

If you buy an engine, get it from the dealer. I can't think of an operation out there that knows how to build them correctly, en masse, that will ship to your door. You'll also have a 2 year, unlimited mileage warranty.

best, whit