Just fitted a propex myself, both Ebers (diesel) and Propex (gas) go for silly money secondhand, I ended up stumping up an extra £75 and just bought a brand new one from http://www.justkampers.com/search2?location=shop&method=match_all&keywords=propex
They were much cheaper than anywhere else, quite a lot of their stuff is priced very competitively.
Any non-room sealed heater would be a bad idea, (both propex and eber are room sealed, fumes are exhausted out through the floor) you would get back to the van, jump in a close the door and not realise you are being swiftly poisoned to death by monoxide. It doesn’t smell and just makes you sleepy. Also, burning gas creates condensation.
Probably be cheaper to get one of those posh car alarms where you can set the car to start up in the morning by itself to defrost, although it will cut out if someone tries to drive it off, I wouldn’t want to leave my van ticking over by itself because thieves would still try and take it.