Well this is interesting.
First off, it's doable. Anything is, really.
Here's your swap list:
engine and cradle
complete harnesses - engine and instrument panel
computer to match the drivetrain
You could use a GT or XT if you can't find a wrecked WRX. Same engine.
You can blow the EZ30D (been there, done that, drive it daily and get more HP than a WRX). On this forum a member has a Raptor SC for sale, WantAnOutback, and he's in Cali. You just add in 650cc WRX injectors with a modified fuel rail, higher volume fuel pump, adjustable fuel pressure regulator, #7 spark plugs or higher, WRX MAP sensor (rescale the ECM), a shut valve for the brake booster so air isn't pushed in to it, intercooler of your choice, (I recommend a WTA either way you go, works great), move the ABS module and PS reservoir, and tune it with help from Ed at XRT. See my thread sleeper vdc for more info. Lots of power and you keep the automatic. It would be a good idea to swap the center diff to a VTD from a JDM or VDC car though. The LL Bean has the MPT differential and you'd get a lot of front wheel spin on take off, plus the VTD diff is a tough set up that will put up with the torque transfer. Just ask Forester owners that have modified their engines.
You could find an Outback with a 2.5 and that would reduce the need for an H4 interior harness. You'd still need the WRX engine harness and comp. Then you could swap all the leather, trim, etc.
Get a Legacy wagon and use that as a platform. It's already sitting lower than an Outback, you just lose the lower paneling and flared fenders.
Tuning is easy. You'll need a laptop, Rom Raider, ECU Flash, a Tactrix cable and I would recommend wiring in switches in place of using the jumpers, you wouldn't be up and down getting under the instrument panel to connect and disconnect the jumpers when reading and flashing. I set up 2 toggle switches between the steering column and fog/cruise/wiper heater switches. Works great.
Cost difference between boosting your H6 vs swapping in a WRX drivetrain is huge. The swap would be higher in $$$ and labor time.
The H6 will not need any engine modifications other than the bolt ons I mentioned. You'll just have to make sure there's no vacuum leaks, no valve issues, good compression, the car isn't burning oil in any manner, all the oil seals are good and you have a good thermostat. Make sure the cats are good and not constricted. You won't be spending time in swapping drivetrains and harnesses. You won't need to buy a whole car to park next to the Bean while you swap parts, then have to get rid of the shell of the donor after.
I actually spent about a week of evenings on my car (about 3-4 hours a night) initially getting everything modified after I had all the parts I figured I needed at the time. I had a shop with a lift and all my tools, too. Then we started tuning and had the pop ups here and there to take care of. I spent 8 months prior to starting figuring out everything with most of the time being spent on the computer tuning portion. At least that part of the equation is done for you.
There's probably a few things I am forgetting to mention, but it'll come to me as you have questions.
It's great getting in to modifying any car. I've done work on all kinds and types of cars over the years and it's addictive. You'll never have it "finished", always coming up with something different. Figure out your cost variables, a budget and the time you want to spend on it then make your decision. Either way, it'll be yours.