That APU on eBay - looks like a lot of accessories are missing.
If you want a cheap air producer look for a timex military surplus unit like the Microturbo from the Hawk and other aircraft which I've seen advertised for around 1,500 GBP as recently as last year (I think it was Everett Aero in UK).
It's also in your required weight range.
I think the only problem with the ex UK MoD Microturbos is they are usually missing the electric starter.
I sold some ex Indonesian Air Force Microturbo air producer cores for about Euro 1,000 each last year.
EDIT ... Everett have some in stock
www.everettaero.com as well as some other alternatives.