there is a comapny called
Varitech that many people also use, they are based in north devon i think it was, they offer a systems, fitting service and a hot system too.
http://www.varitechcatalogue.com/As with anything, you also need to check up how long and how much consumables will cost/last! Stuff like Resin, RO membranes and pre filters.
If your using Ionics, you need to use their own resin chambers which are something expensive! whereas most if not all others manufacturers of van systems use standard DI cannisters.
bear in mind theys guys with sales will pretty much tell you what you want to hear to grab a sale.
Take yell.com - first thing they ask is what type of market are you in, Commercial, Residential - then when you tell them, they will blab on about how many hit they are getting of that particular type of business.. its easy to spot! they ask questions to hook you on the line... be aware of sales from the start and dont be pressured into anything.
Any offers they make, keep a a note of them, if they decide to take them off or give you a ultimatum say "sign up today and well give you 2 free pole" dont, unless its really what your after - you can try going back to them and say, well im considering going with either you are them, however if you agree to put those 2 free poles in the deal i'll buy my system from you..
@ bridger, the protective coat is sent to you so they only have you van for a day to fit the system! you can always ask them to do it for you, however they will need you van for like 3-4 days for the paint to dry off, before they can install your system.