I have some reservations about the use of an APU. Firstly they are not generally designed for very long running lives - in most aircraft they are shut down once the mains are running. Secondly, they operate at very high speeds - generally not far under 100,000 RPM. The noise and gearing implications of that are problematic. I certainly remember doing some work on the Saab Gripen where my perception was that the APU generated 4+ times as much noise as the two main engines at idle - certainly you couldn't be within about 50 yards of the aircraft whilst the APU was running without ear defenders, whilst you could be stood next to it (albeit in some discomfort) without whilst the two mains were running.
The Aero Adventure engine is rated at 65,000 RPM which is still very high, but at-least a factor of two below, and one assumed more amenable to soundproofing.
G