Each option has a win and a loss.

The version 6 sti motor gives you the good heads (win) but the slightly lower spec bottom end.(Loss) And I would expect most of these are getting a bit long in the tooth as they came out of a 2000 model car at best. Might find its tired and needs a rebuild.

The later model STI has the tougher bottom end so you win here but lose on the AVCS heads which you cant use to best effect without an ECU to control them. Maybe see if you can retain the heads and put in non AVCS cams and running gear. (I am sure you could will just eb a case of how much....) If this could be done at the right price I reckon this would be your best option.

Speak to someone who knows over there and let me know if its a goer. I have a good contact over here who should be able to get you the motor at the right price