I'm doing a lot of thinking/planning right now for my Subaru swap. That engine is capable of poutting out 260-280 HP stock. The single turbo version is very similar to the WRX engine and puts out 220 or so. Currently thats the engine I'm looking at using.
All of these are JDM (Japanese Domestic Market) engines and as such are not smog legal here. Which is great because they are cheap! I went to a place today to look at engines and you can get the single turbo engine for $650, $950 for the twin turbo, or $1250 for the Japanese WRX engine. The downside is that they typically don't come with an ECU or wiring so you have to start from the ground up.
http://www.nippon-motors.comFor the twin turbo engine there's a lot to do. Fiid, a user here is doing a twin turbo installation and his thread is interesting reading. He ran into problems with the starter motor interfering with the second turbo, and it looks like the exhaust construction will be tricky since the turbos are sequential.
As far as I know, KEP has adapter plates/flywheel combos for most of the Subaru engines.
I figure that to do a conversion like this, you need to figure maybe $2000 to $4000 depending on how you do it. I believe that a large part of the cost/work will be in the engine management system and wiring.
This place:
http://www.outbackmotors.com/ has some nice motors that put out a lot of power. Also they have some good resources for wiring harnesses and engine management systems.
-Tony