The V12 of a 1997 BMW 750iL will have around 400hp in stock form, but...
-There's probably a performance chip that you could buy to "fix up" the V12 a bit.
-Bentley's Publishing sells a good manual for 88-94 7-series cars which includes good info about the V12. Probably the 1997 version of the engine is almost the same.
-Be advised that the engine is really crammed into the available space and is quite difficult to work on. Also, there's two of everything: two air intakes, two mass airflow meters, two engine control computers, etc.
-Remember that you're talking about one of the most complex cars ever built and there's a lot of things on it, which means there's a lot of things to go wrong, but the engine itself is very reliable and provides one helluva driving experience.
I'm not trying to talk you out of it, just trying to make you examine your goals. If you want to drive one of the best cars in the world at an affordable price this may be a great opportunity for you since you likely have the skills/guts to do a lot of the maintenance yourself. If your goal is to have an engine to tinker with you should probably pass on the 750iL.