The 4 is the one to go for, its the first car to have the 276bhp and AYC but the AYC units on the 4s and 5s were known to be faulty so watch out as they are around 600+ quid to replace.
These cars need super unleaded fuel and octane booster, they drink the stuff so watch out for that, also, servicing costs are very high, no matter how much you look at it, its going to rob you rotten, especially parts like brake discs which are brembo items.
If you can afford it (look at the can part of it as i earn 30k per year and still struggle sometimes running the impreza) then get yourself a run around (ie a 205 diesel) which is cheap to insure, good on fuel and cheap to service, you will save more than you think doing it this way rather than running the evo all the time
Tony