Paul Griffin did an excellent set of replies to your questions, pretty much spot on.
All of the manufacturers do decent trolley systems, Aquatech, Cleantech, Ionics, Omnipole, they all do what they say on the tin.
All will have their advocates.
A van like an Escort van is ideal for a trolley system, but you will be restricted by the amount of water you can carry with you if you go for a R/O type system.
DI can work out more expensive, particularly so if you are in a hard water area, you will go through resin like there is no tomorrow!!
There are many different solutions to arriving at the system you want.
If you have a tow bar and trailer you can use that to carry extra water or your trolley in, you don't necessarily need to buy an expensive (relative) custom built trailer.
You should see the Omnipole trailer, top piece of kit but a big hole in your pocket. The Ionics one is good too, smaller though and likely to run out of water (depending on the work you do)
Aquatech do a first rate, no frills trailer, ditto cleantech.
But you have to dip fairly deeply into your pocket for even the cheapest setup.
You also of course have the back pack option, starting to take off now, with a couple of manufactures doing them.
You certainly CAN make up your own system, check out Matt's postings on DIY systems, first rate guide
But if you do buy a custom built system, if things go wrong, or you need advice, they are there at the end of the phone for you.
Through the Ionics site, you can find links to training courses in WFP, not expensive and you will learn how to use it correctly. Shortens the learning curve, and believe me, its steeper and longer than you might think!
Simon Carter also makes some very good points.
Many trolley users (myself included) end out adapting their equipment into mini-van mount systems, this allows you to mix and match, you chose the quickest option depending on the account you are working on, for me that means 95% of the time I am using it as a van mount (I work out of the back of a hatchback) I think if I had a van (even an Escort van!) I might use the trolley a little more, it would be much quicker to take in and out of a van as against a car.
I think that'll do from me
Regards,
Ian