[WFBowen]

Tim:
I'm not an expert, but here are my suggestions. The easiest route would be to find a complete 912 motor and bolt it in - no fabricating or guessing, etc. You can put a 6cyl 911 motor in, but there are numerous detail changes you'll have to make (oil tank, gauges, etc.) As for the other motor options, a type I upright VW will bolt in without a lot of work and these motors can be built up for small amounts of money. The Type IV you mentioned is the late VW/Porsche 914/912E/VW Vanagon "suitcase" motor. Installing these involves some work - this has been covered on this thread in detail. Like the type I, there are lots of relatively cheap aftermarket parts for these.

Final thoughts: If you're buying a "running" motor, make absolutely sure you hear it run and rev - if not, consider it a basket-case motor even if it's beautifully detailed and plastic wrapped on a pallet (unless you know the PO or it's at a legitimate shop.) Should you go the 911 route, I'd avoid the 2.7 and look for a 3.0SC (JMO.)It all boils down to how much you can & intend to spend on the project.
Good luck.
Bill.