My first 3 planes were trainers and the same engine was on all 3. My unofficial 'second plane' (first non-trainer anyway) had a different engine and the trainer was still flying at the time.

I think having your trainer, or other plane, ready to fly is a very good thing no matter how much or how little the engine and electronics cost. I currently have 3 planes ready to fly. Two of them have four stroke engines, so there is a bit of money tied up in stuff that may not go out very often.

You can always buy a cheaper engine and put it on the trainer; and move the AX to a more advanced model.