I absolutely recommend the turbo. It can be done for right around $2000, give or take (depending on how resourceful you are) a little. The parts are out there and they can be had. Shoot, people are getting rid of turbo's for as low as $200 (REX TDO4) and WRX top mounts for the same. Add another $500-$700 for the crossmember, exhaust manifold (headers), uppipe and full exhaust. Thats practically all the turbo related stuff you need. Tack on $150 for a RRFPR, $120 for a boost gauge, $150 for a new Walbro 255LPH fuel pump, and another $200 or so for a used S-ITC (Apex ignition timing controller) and thats most of it!
That there adds to right around $1700, so you have $300 for misc stuff. Trust me, there will be lots of this, so have some extra money. It can be done, no doubt about it. Lots of help can be had right here on the boards also, give it a shot!
Or, you could invest the money and try to make a little more so you can buy nicer parts for the kit.
I didn't forget the clutch. If you keep it low boost (3-5 psi) you can get away with it, though it might slip OCCASIONALLY.
You absolutely must have the turbo manifold, like the ones' found on a 1991-1994 Legacy turbo or the US Spec WRX. There is a great difference between the stock N/A headers and the turbo headers (exhaust manifold). Once you have the manifold in place, you will need to bolt your uppipe onto that. Let me explain one thing about the uppipe to you: for an EJ25, when doing the Legacy style turbo setup, it is recommended that you get a steel spacer to put in between the the bottom of the uppipe and the exhaust manifold. Since you have a 2.2, I am not sure whether or not you need this, as the reason for having it is so the uppipe clears the passenger side heads of the 2.5. I think you will be O.K., but I am not positive. If the uppipe doesn't fit, you will need to have a custom one made or go to a machine shop and have them construct a spacer for you, it recommended you use one at least 20mm thick. I have one on my car and my Legacy turbo uppipe works just fine (no cat!)
Your list seems to be complete, the only things you need are oil feed and return lines, and some coolant line to the turbo. You can get the oil feed and return lines from www.autocaresubaru.com. I use the banjo-bolt style return line, which requires no tapping of the oil pan, and works just fine (no backed up return line). You may run into some trouble trying to find the right voltage zener diode, so it may just be in your best interest to get the unit from Split Second, it's like $70 (zener's sometimes have trouble in different weather conditions).
What kind of exhaust do you plan on using? What kind of turbo, intercooler? Do you plan on having any sort of timing control so you don't detonate/ping?
One piece of advice I can give is take your time. If you plan on installing the kit yourself, it will take many of trips to the hardware store, and quite a bit of self ingenuity to get it to work right (just figuring little things out, like how to make a brace for the turbo so it doesn't move around).
Oh, one more thing. You might want to get a fuel pump. The WRX units flow plenty of pressure for 5 psi, so just pick one of those up for like $40 in the classifieds on i-club. The US Spec WRX pump will drop right in.
Don't buy a Vortech RRFPR, get a Bell Engineering unit, much better. Mine works O.K., but it is nice to have a unit that responds to each INDIVIDUAL lb. of boost, instead of just 4 psi.
Don't forget all of you gaskets, that is a pain to have to find when your ready to install, so get that out of the way. Oh, and since your going to use the WRX exhaust manifold, you will also need the WRX front sway bar, or else you will have nasty squeeks.
What year is your car, are you sure it is MAP based?
Well, I haven't really thought about doing that, but it seems like a good idea. The only thing would hold me back about doing this was the install of the Legacy turbo setup. You really need a lift or some good jacks to change to the turbo crossmember, as that is the hardest part. After you get that on, its mostly just bolt on of factory Subaru parts, such as the uppipe, turbo, and exhaust manifold. I think a kit is easier to install, but isn't as reliable in the long run.
The one thing that leads me to believe it would be a popular kit is the extremely low price. If I could assemble them and sell them for less than $2000, I think people would be interested.
Kevin, I was not sure if you could use the WRX crossmember or not. I would see that as a viable alternative, and since it is cheaper, why not use it!
I am pretty sure it is the same thing as the Legacy Turbo crossmember. The only difference between a turbo crossmember and a N/A crossmember is a little indention on the passenger side of the unit to make room for the downpipe to fit. If you had the means, you could probably just bend the medal yourself to make room for the downpipe, but it might be kind of hard.
One more thing. Blitzkrg, the guy who you say is using the WRX crossmember, also did the EJ20-T conversion. So, I guess it would fit then, go for it.
Although he did say he already has rims, so I assume that means he has new tires too, but one never knows. (Could just be new 16"ers with RE92s...)
After that, I say go for an 18 or 20mm STi rear swaybar (no bigger), with maybe some Kartboy rear droplinks and front inserts. Following that, save up a bunch and get some STi strut top hats to go with your new suspension (i.e. STi struts & springs, or whatever).
And a four-wheel alignment after the changeover. Then you're in good shape.