I fitted a Webasto (water type) in my series as its a camper. There are a few things to consider:
You should not be routing a fuel line inside the vehicle and the same would apply to any extra tank.
You should have the combustion unit outside the vehicle if you are sleeping in it in case it catches fire. They are usually in a metal box under the floor on trucks and campers and in a vented box on narrow boats. My water one is under the bonnet but we still have a smoke alarm.
The exhaust must be mounted where it cannot get back in. This is not as easy as it sounds (if it sounded easy!). Its fine if its windy, but a calm cold day is a problem as it can casue a cloud around the vehicle. Mine exhausts just behind the N/S front wheel and we take care to check the wind direction when parking up for the night, but to be sure we also have a battery CO alarm.
What does for the air ones is getting water in and seizing the fan bearings, this is common because they are often undeneath the vehicle and not well protected. A benefit of the water type is you can mount it high under the bonnet with only hot water coming into the passenger / sleeping compartment.
I can't say about the cheap Chinese ones (I too have read favourable reviews) but Webasto have a detailed set of installation instructions covering fire, fuel and CO that require a certified installer - essentially they treat it like a min-diesel combi-boiler which is what the water type is. I bought second hand (£75 vs £900!) and downloaded the instructions.
My question would be "why do you want one?" If its to pre-heat then get a water one as the engine will be warmed up too. If its for camping / sleeping then don't mount it inside. They shut off on low voltage but from experience I would say they work best with a leisure battery of at least 100ah. We have 100 ah and its a bit marginal. I find the Webasto draws rather more current than the spec. I think the spec is one start then run, but in reality they cut in and out so you get a lot of starts and the ignition is a glow plug that draws 7A at least. The water one is worse because its running a fan and a water pump then also an auxilliary heater fan in the cab.
We wouldn't be without ours to the exstent that I always have a servicable spare ready to swap in if we are going camping, we have had a good night's sleep with an outside temp of -2c and been able to wash and dress without freezing. (I didn't say "in comfort" because there's no such thing in a Series!). The other thing is they are noisy, a bit like a mini-jet engine which is another good reason to put them outside if you plan to sleep.
