The weight difference between a stock 2.2/2.5 sohc Subaru and a Type 1 aluminum block, dual-carb VW is about 40 pounds over the rear. Figure another 30-40 pounds elsewhere--mostly forward*--for the radiator and etc. The Suby is lighter than a Type 4.
The engines to have are:
1. 1992-1995 2.2. (Easiest install, non-interference valve train)
2. 1996-1998 2.2 (mostly the same, but an interference valve train. Slightly more power.
3. Later 2.5 with 2.2 sohc heads and intake (the so-called "Frankenmotor," which, with a set of cams and a step larger injectors will make about 180 hp** at the crank.
The dohc engines are a little wider and a little heavier--not a problem unless your frame rails make it so. And a lot of earlier and pan-based Speedsters may present a fitment issue.
As advised above, if you've got the dough, Carey is a very well-respected builder who knows way, waaay more than me or just about anyone else here who might chime in.
*IMHO: rear-mounted radiator schemes on these cars are a good way to make a complicated mess from what would otherwise be simple and functional.
**IMHO: if you need more than 180 hp you're going to make yourself some expensive engineering challenges. Turbos work great but make a lot of heat, and getting rid of the heat quickly becomes difficult. Not impossible...but difficult. The cost/hp curve in these cars gets pretty steep once you get over 180 hp.