Subaimp

Not sure where to post this but here goes .....

Hi Guys, totally new to Subaru so will apologise in advance for the possible silly questions.

I have what I believe is a 1994 Subaru turbo engine, its marked up EJ20, it has what I think you boys call the low rise std inlet manifold, how do you differentiate between a normal turbo motor and an STI ?
Am I correct in saying that an STI motor has forged internals and is therefore better from a tuning potential ?
Is it possible to swap for any other Subaru engine without loads of hassle ?
I have what I believe is a std ecu which is a grey box with a blue and silver sticker marked 7D
The car has "normal" brakes and solid discs (akebono system ?) as opposed to the bigger front callipers with subaru cast into them.
Basically I don't want to spend money on the motor if I don't have the ability to go beyond say 270 bhp.
The engine is an unknown quantity which I need to remove from the car, it runs fine most of the time (Sometimes pops and bangs and runs rough then on next start up its as sweet as anything) but does not circulate water despite the thermostat being removed and checking the water pump turns.
Original plan was to run the car as is and see what we have, then remove to check over and sand blast, Im now thinking just to remove engine and replace with a proven engine (sti or tuned) from here or similar, but don't really want to change ecu and wiring loom, I'm assuming if for example I went for say a 2.5 engine i need to change ecu and wiring harness.

Sorry for all the questions and thanks in advance for any help.