I have one of these machines with the external heater, great bit of kit and for the money best value I have ever had from a portable, Vernon's been great with service too, set out a replacement wand imediatly as courier damanged first one during delivery.
The Tech that uses this machine the most, says that the heater is definatly better than the prochem one because it puts out more constant heat, it's been in daily use for a month now and no problems, to early to say how durable it is yet but a its a good start and I have had much worse with other machines.
As for a 135psi and 2x3 stage vacs with hoses and tools for £999. i'm having one as a back up! ( and if your reading can i have a discount Mr Purcell after i said all those nice things!) Mainly because that spec on anything eles would be at east £600 more
Oh and the stair climbing wheels are good too!
Only down side, it could do with a bracket/holder for the heater like the steampro, but as he makes them he'll probably have one this time next week,