Subaru "classics" had different internals and blocks to the "new age" 2ltr cars, which have stronger rods (big plus side for the new age cars) hence why they can run 400+bhp all day when well maintained
New age JDM cars (twin scroll, THE best petrol engine subaru have produced to date) are different again to the UK 2ltr engines, with revised cranks (drilled/nitrided for better oil flow/strength), quick rack steering, rear wash wipe and lots of other little goodies over UK cars that make them a more desirable car to own (though people will try to put you off saying they rev their nuts off on the motorway at 70), 3k rpm isnt too bad tbh.
You also have some differences with the diffs, for instance, it was only the classic STI type RA that actually had a helical/Suretrac type front diff standard, it was open on virtually all the other classics, on 2003 onward STI's (especially JDM's) it was an AP suretrac type, the 2005 changed back to helical, these new age STI's also run with a R180 plated rear diff, same as the later version 4-6 STI type R/RA's (maybe also the standard classic 4-6 sti, would need to have a look).
Tuning wise, your average UK 2ltr is capable of around 350bhp on its standard turbo with a remap/exhaust/fuelpump upgrade, your JDM will be a little higher (twin scroll) on average with more torque (370-400lbs) if you get a good mapper
Then your going to have to choose a spec, its fairly easy with an Evo, GSR or RS, the RS being more "hard core", with the MY03 onwards STI's you have (say 2003-2004 cars including the WR1 and PPP cars), your bog standard UK 265ps car, your PPP'd 305ps car, your 320ps (PPP) WR1 (note it comes with A-DCCD which was only available to the Solberg edition which the WR1 is based upon), then you have your JDM versions, the KL or "poverty spec", jap import, doesnt have A-DCCD so its best avoided, the GL which does have the A-DCCD, the Spec C (which all the later MY cars from 2005 based the geometry/wheelbase setup on) which is the lightweight and quickest version, in 3 versions between these dates, the 16 inch motorsport (just dccd, no auto mode and depletes several other parts like engine oil coolers, 16 inch steel wheels,hence the 16 inch version and not very decent throw away justy type interior,) its basically a car designed for stripping down, the 17 inch version with the really nice BBS wheels (also available on the drivers pack for the GL), no electric anything/central locking etc but has all the goodies like oil coolers (front mounted type), and the Limited verision which didnt have the BBS wheelsbut had a ncie black interior, passenger sun visor and electrics/central locking.
There are lots to choose from, some are a lot rarer than others and you have to know what your looking for, but great cars, whooped butt against the evo 9 on a photo shoot for evo mag greece back in 2005 when the "9" had just come outand lots of good advice from the likes of Edd and scooby there too
Tony
tl;dr
dont buy red cars
) but if you can drive a JDM twin scroll I think you will be sold