RE: Helicopter Engines?
The heli engines actually work nicely --- as you noted, the better ones are timed for higher rpm these days.
I sympathize with the money vs wife thing.
But keep in mind.... to your $200 heli engine you will have to add a $70 tuned muffler or pipe setup to truely take advantage of the power and develop the rpm and torque needed for the aero application. Without the exhaust system --- you might as well be using a TT46pro or OS50 aero engine to start with. Plus for most heli-engines you will need a special shaft-extention nut - the output shafts are often too short for a prop hub... thats another $10-20.
Retired or re-built heli engines are sometimes a good value. There were a bunch of YS80 and OS70 engines floating around fairly cheap for a while. With the right exhaust system and suitable prop .... they have some good higher rpm/speed potential.
Speaking of which... since you are on a budget... one of my favorite combos.... TT46pro and Jett-stream.
For most .40-.50 size Q-500, deltas, shrikes, etc it is a performance/$$ setup that is hard to beat.