I've had one for 8 years now and it's been superb its done just over 2000 hours and still runs great, saying that I've got a spare one with 3600 hours and that runs fine as well.
Things to look for:
The heating is quite complicated, so make sure the actuator rod goes up and down with the thermostat dial.
The ignition switch can burn out, I've been through a couple.
Check the exhaust for leaks as they're flexy pipe they tend to break at the joints.
Other than that its just an engine, blower, cat pump and heat exchanger so just check for leaks, and make sure it runs well.
They're pretty robust, so I'd recommend.
Speak to the guys at Alltec they know them inside out.