I'm no expert but best advice is give them a test drive and see what you like best.
There are subtle differences between versions and I don't know enough anorak stuff to tell you what's what.
What I will say is, You will hear a lot about ringland failure on the 2.5's. Whilst in my opinion it's a fair amount of scaremongering there is some truth to it.
Whilst this was an issue with a small percentage of cars it was down to people putting too much power into the standard engine, poor maintainence, bad mapping or ragging the nuts out of the car. If the car your looking is well maintained, well set up and hasn't had it by now then as long as you maintain it properly there's no reason it shouldn't carry on.
If the engine has been rebuilt then as long as it was done properly again no issues. Although it would be wary of a rebuilt engine and would want a proper inspection before buying.
Don't get blinded by the rally look stickers on cars, do the proper checks and if poss get a local Subaru specialist to do an inspection it rather the RAC/AA
Being from Surrey you are lucky in that there lots of scoob specialist in the area. Dan at Surrey Subaru specialists is very good
There is also TFS, GRD, Jolly Green Monster (one of the best mappers in the country) Proven Subaru (Dealer) just off the top of my head.
I would recommend you join up at http://www.surreyscoobies.co.uk/forums/index.php
It's my local club aswell and they are a great bunch of guys who are all local to Surrey. We meet every 2nd Sunday of the month at the Fairmile pub in Cobham. Your more than welcome to pop along Sccob or not.