I'm only just moving into guttervac, so I'm commenting here at a very early stage and based on the research I did before buying.
I've bought a Pramac S5000, 5kVA. Honda engine. Mine was off fleabay from a guy who had realised as he fitted out his catering van that it wasn't powerful enough for his what he needed to run from it. There are some bargains on there, but I also suspect some utter lemons, so tread carefully.
There are cheap generators out there. They are cheap for a reason. The research I did suggests that the Chinese ones seem to last for a short time then fail, so I suspect you'll buy cheap and buy twice. I chose to go the second hand route because the seller seemed utterly genuine (and was!).
This is the one I was looking at before I fleabayed:
http://www.justgenerators.co.uk/pages/HondaEC6000.htm which should be up to the job.
You're not going to want to drag it in and out of the van, so a long heavy duty extension lead is a good purchase. If you're going for a 110v vac, you'll need 4mm cable (which is damned heavy!) and you'll need to make up your own, as you don't appear to be able to buy them longer than 14m made up. I've been making mine this weekend - I've made two 25m extensions. 240v is a lot more forgiving on cable as the current is lower.
If you're leaving it in the van, either you'll be leaving the back open or having a hole punched so you can route the exhaust out of it.
Hope that helps.
Vin