There's a nice list of recent (roughly 1990+) engines on nasioc.com. Select Forums -> Subaru Conversions->READ FIRST.
To "paraphrase", :
QUOTE(nasioc.com @ sometime)
EJ18: Found in Imprezas from 1993-1997. Rated at 110hp and 120 ft-lbs
EJ22 Phase I: Found in Legacies from 1990-1998 and Imprezas from 1995-1998. Comes rated at 130hp and 137 ft-lbs, 135hp and 140 ft-lbs, or 137hp and 145 ft-lbs.
EJ22 Phase II: Found in Legacies in 1999 and Imprezas from 1999-2001. Rated at 142hp and 149 ft-lbs.
EJ22-T Phase I: Found in Sport Sedan and Touring Wagons (Legacies) from 1990-1994. Turbocharged and non-intercooled. Rated at 160hp and 181 ft-lbs. Closed deck, cast pistons, forged rods, oil squirters: turbo motor. Poor flowing heads.
EJ25 DOHC Phase I: Found in Legacies from 1996-1999, Imprezas in 1998, and Foresters in 1998. Rated at either 155hp and 140 ft-lbs in 1996 or 165hp and 162 ft-lbs in 1997-1999.
EJ25 SOHC Phase II: Found in Legacies from 2000+, Imprezas from 1999+, and Foresters from 1999+. Rated at 165hp and 166 ft-lbs. There was a slight compression ratio change from 1999-2000 where it went from 9.7:1 to 10.0:1.
USDM EJ20: US Domestic Market WRX. Found in 2002+ WRXs. Rated at 227hp and 217 ft-lbs. Turbocharged, intercooled, open deck, and decent heads.
EJ25T DOHC: Available in 2004 in the Forester XT. Rated at 210hp and 235 ft-lbs. Turbocharged, intercooled, VVTi (Variable Valve Timing), and drive by wire.
EJ25T-STi DOHC: Available in 2004 in the WRX STi. Rated at 300hp and 300 ft-lbs. Turbocharged, intercooled, AVCS (Active Valve Control System), drive by wire, intercooler sprayer, and semi-closed design.
There is also a large variety of engines available from the JDM and EDM locales. The EG33 from the SVX and the EZ30 from the new H6 Legacy can also be found, but they are larger and will be more difficult to swap.
Anyway, it sounds like you might not want to spend top dollar, 'cause you said you're "not inclined to spend top dollar". I'm thinking that one engine to consider might be the EJ22 (2.2 liters). I don't know much about phase I vs Phase II, but these are the engines that made the new generation of Subarubi what they are : strong, reliable, powerful. Not a ton of power compared to WRX, STi, etc., but it's a nice bump up in HP for a teener, and these cars are everywhere.
Two things I can say from personal experience : don't buy from a stranger on Ebay unless you really, really check them out. (I got hosed.) And, make sure you buy a known running motor. I bought a "builder", and the cost of parts turned out to make it not worthwhile. For the money I
wasted invested in education, I could have had a really sweet engine.
I'd say if you're going the EJ22 route, just look for a complete car (Outback, etc.) that runs, but looks like crap, has a bad tranny, broken windshield, etc.) Figure the engine is good to about 180k miles, and count back from there - you're prolly not gonna put a ton of miles on a teener, so 100-120k is still pretty young. Make sure the head gaskets are good, compression is good, and there's no horrible oil leaks. And park it in a friend's yard.
If you are inclined to go "cheap", as opposed to getting a fresh, young, tested engine ready to bolt in, don't forget about the money you'll be putting into parts, gaskets, extra tools, etc., and time. If you're itchin' to drive, as opposed to itchin' to get another project started, defintely consider putting up a few more $$ at the start.