I have a Speed 3. I believe the Speed 6 is the same engine, but with AWD. The engine itself is pretty reliable, but the motor mounts Mazda uses with that engine are complete garbage.
Another choke point is the OEM bypass valve is weak, along with the turbo inlet hoses. Easily fixed over at cobbtuning.com.
As far as trading the AC for one... I would only do it if you want a vehicle that is comparable in performance but want the extra two doors. How many miles (kms - sorry... force of habit

does the Speed6 have?
As far as pricing goes, there is no such thing as a "straight trade" in the negotiating game. Negotiate the value of your car, and negotiate the price of the one you want. Do your homework online to get a ballpark for those numbers. Just want to make sure you don't get boned by a dealer.