Hi Pete
The residential kit is perfectly OK for awkward windows like you mention. The only downside is that it will only produce a limited amount of water before you need to replace the cartridge. So if it is occasional use then you you'll find it's much better than a fixiclamp and scrim and is worth getting.
When vistech says they use it on commercial work I suspect they're not actually referring to the residential kit (which has to be connected to a tap when in use, by the way).
Regards
Mike Boxall