Numbers I came up with were about 97 to 112hp for the 22R and RE variants and anywhere from about the same to 140 to 170hp for the TDI depending on the model year. My parents' Passat and Golf TDIs get about double the mileage of my pickup, but I'd doubt the pickup would get the same mileage as a VW even with the same engine. Still, if you could get half again as much mileage, roughly the same horsepower, and more low-end torque it would still be an advantage.
Yea, I too wondered about that engine in a heaver vehicle. I was thinking a 98 vintage 1.9l juiced up may do the trick. I also wonder about the 99 - 2002 TDIs and how much computer stuff to deal with. There is a guy on TDIClub.com that running low 14s in his TDI.
For ease of conversion, you want the 22r. The v6s are more complicated. And most are doing this with the pre-99.5 engine as the ALH/BEH TDIs require the engine, harness and ecu IIRC. With the older ones, the engine is enough. There are a few threads over a the TDIclub too.