You can put whatever you like into it.
EJ engines are much of a muchness in terms of complexity. They'll all bolt in with minimal hassle. The real issue is how you are going to do the wiring. You'll have to merge the engine (donor) and your BH5 wiring harnesses.
The easiest, low-hassle swap for you (RHD) is a JDM TT (RSK, GTB, etc) front cut of the same year/revision (A-C vs. D/E). Wiring is simple plug and play (remove engine harness from JDM donor, replace yours, no merging of harnesses required). You'll have to pull out the whole dash, including AC and heater, to get the harness out regardless of what engine you choose.
What's 'the best'? - depends on what you're looking for (low grunt, laggy but whooshka power, driveability) and what's available. JDM means 2.0L. STI's are good and solid. You could also use a JDM forester, and other engines.
You'll need a turbo engine crossmember. Your gearbox and drivetrain will survive ok if you're sensible - but don't expect to be able to give it a hammering without busting something.
Safest (and cheapest) is to get a JDM front cut with engine management, wiring etc all in place, with gearbox and rear diff; or find a local wreck with an engine you like the look of. Easiest if its R160 rear diff, otherwise you need to muck around converting shafts &/or hubs.
Don't make the mistake of buying just an engine; you'll find all those other little bits and pieces you need will cause a big headache and end up costing you way too much.
Check out subyclub.com - plenty of RHD BH/BE legacy/outback conversions of various types. TT, JDM forester turbo, STI WRX, etc. clubsub.org.nz also has lots of JDM goodness, as NZ has an open door policy on imports.
What's your fuel like in Sri Lanka? Be careful not to get a JDM engine that needs 98RON if your local fuel is not up to par.
Also consider if you want to have the ability to tune -- JECS ECU's (early A-C BH/BE TT's, Imprezas of the same era) aren't as easy to get tunable, though there are solutions such as ECULABS.
Here's my
simplest case TT conversion of a Rev B BH5 outback. Didn't swap anything in the drivetrain.