I've been doing a lot of reading and research on an 02 Forester build I'm working on, and I think a lot of my research would apply to you as well.
Firstly look over some completed builds and find out what steps people are taking for your chasis and model (if you can find someone who's done it that specifically). I found a few very informative builds on the Forester forums that clued me in on what I needed to expect and the forks in the road that I would encounter.
Another question to ask yourself is how big do you want to go? What are your goals for the car, and what's your budget like? I struggled with this a lot, and still do, but I've moved past a lot of my basic decisions (transmission, motor, AVCS, turbo size, etc) and I'm into the nitty gritty mostly now (oem radiator or aftermarket, MAP tune or MAF tune, etc)
I can certainly help outline some of the information I've come across in private messages, but asking a question this open is going to yield a lot of vague responses. Like this one.
My two basic schools of thought on doing my conversion were:
-keep it simple and go with almost entirely OEM parts
-go nuts and build for power while you've got the motor out
If you want to go with a basic OEM WRX swap you'll probably have an easier time, as hybrid builds, wiring, fueling and all that stuff will becoming more complicated the deeper you go. If simplicity and cost effectiveness is paramount then I'd be in the market for a smashed low mileage 02-05 WRX to rob blind of it's parts, upgrade to a VF series turbo and call it a day. This option could be done for maybe $5,000-8,000 depending on the car your source and the amount of labor your do yourself. Output will be maybe 300/300 with a VF43 turbo upgrade and will be moderately reliable depending on the starting mileage, use, tune, and transmission condition.
If you want to go nuts and have the money to go nuts then start reading up on AVCS swaps vs hybrid swaps, transmission choices (built 5spd vs 6spd?), and turbo choices (big 35r or 18gxtr?). This option for me is panning out to around $15,000-20,000 all told based on my build list (I have an extensive list of parts I've purchased and parts I have yet to purchase). Output on this depends on however deep you really want to go, 450/450 could be approachable for the upper end of the price point above. For me the build of the motor and the Sti transmissions durability record was worth the extra money; If I'm spending 8k, why not spend 15k and get the extra power and durability if you have it?
Anyways like I said, PM me if you want to talk more tech, or share links or anything. Right now I'm being general to just give you an overview of what I think you're getting into.