Unless you intend to get north of 350 horsepower there's not that much advantage to a closed over an open deck. There's a good description of all the Subaru chassis and engine codes here:
http://www.toronto-subaru-club.com/forums/...ead.php?t=84585EDIT: And more on the different engines:
http://en.wikipedia.org/wiki/List_of_Subaru_enginesThere is actually no official EJ20T engine designation, that's just shorthand for an EJ two liter engine with a turbo. The STI EJ207 engine (the first five digits of the engine number) is different than the EJ205 WRX motor because it has all forged internals, pistons, rods and crank, for higher stock boost and more horsepower (227 for the WRX stock, 300 for the STI). That means the STI engines are more rare, pretty highly sought after, and as a result more expensive. The non-STI EJ205 WRX engines are excellent and what most people use for performance conversions. No issues.
Once you get the cats and restrictive intake off the WRX engine, and hopefully onto a standalone ECU, you should get up around 300 horsepower without a lot of work. The WRX engines go for $1000 to $2500. Try to aim at a 2002 to 2005 motor, the sweet spot, but if you're going to do the work and tuning yourself any of the WRX motors will be pretty much as good. The advice I've gotten is that an aftermarket standalone ECU is the only way to go unless you're putting the motor into an older Subaru or other car that has to meet the smog visual check, when you'd want to use the OEM ECU.
If you want to be different and spend a little more the newer Japanese Domestic Market STI's have a similarly good 2.5 liter motor, the EJ257, but what really intrigues me are the NA 2.5 SOHC engines, which are all over the place and for under $1000 are cheap as chips. They're 165 hp stock, easier to install than a turbo, and nearly 200 hp with only exhaust and an ECU, which you'll do anyway to put them into a 914. That's the same non-turbo power as a much heavier and far more expensive 80's 911 Carrera motor, so not bad. But also take a look at the 2001-2004 EZ30 three liter flat six engines, which could be very cool. Also NA, 212 hp stock, 250 with better exhaust and induction, pretty much the same size as the four cylinder engines (less than one inch longer), and with a firing order that should produce an exhaust sound that would be very 911. These aren't the big old H6 3.3 SBX engines, they're much smaller, lighter, more modern and more powerful, the seeming perfect mate to the 914.
As info you'll see a lot of engines at wrecking yards (like www.car-part.com) that have broken cam sprockets from front collisions. The later engines are all interference, meaning that when the timing belt stopped in the collision there was almost certainly valve to piston contact. Usually the valves just bent and a new set of valves and guides will fix things up, but sometimes EVERYTHING gets mangled. So you take your chances with those engines. Luckily heads aren't that expensive or hard to find for any engines except the STI's, so a lot of times you can get an exceptional deal. Not always, though.
The folks at Small Car Performance are great and have all the little bits and pieces you'd need. They specialize in Subaru engines into Vanagons. The folks at Outfront Motorsports are similar, but they put the engines into sand rails, so they're more performance oriented. A lot of good practical performance info on their website, too. Both companies are knowledgeable and incredibly helpful. Good forums are Subbug (Subarus into VW's) and NAISOC (Imprezas).