Be warned - petrol models are THIRSTY. Exactly what economy you get depends on the exact vehicle spec, what payload weight you carry and driving type. The van will do around 30% less mpg on LPG than petrol. A van carrying 1.4tons in the city could give a truly shocking fuel economy figure (say, 10 - 15 mpg). Its diesel equivalent could well give 20 - 25mpg or more. At the extreme ends of these figures, I think diesel would work out cheaper.
Unless you specifically want a petrol/lpg model, it may be more economic to get a diesel and pay a higher price for fuel, but use less of it.
When you look at the van, the things to look for are pretty much the same as cars, but with extra attention to the service history - this really should be done by a Ford approved agent. If there is even one service missing, don't get the van. A simple oil and filter change won't keep a commercial running well, and they can't wait 30 odd thousand miles between seeing an experienced commercial mechanic.
Ask to see all MoT certificates. If any which say that advisories were issued (esp the most recent certificate), make sure you see it. If the problems noted on the advisory haven't been fixed, again, think twice about your purchase.
Also make sure it is the Ford fitted LPG system, not an aftermarket one.
I'm not sure how clutch units weather on petrols - someone else may be able to give some info.