Specifically, I have 3 questions:-
1. How much would it cost to set oneself up for running a WFP system? I would
    add that I would expect the cost of a suitable van to be taken into account. I
    would also like to know what basic equipment would be required to get   
    started.
it cost me 4 grand to get started van was 3 grand 
2. How long should it take for the initial outlay to pay for itself?     2 months for me but a lot depends on the work you have as to how long it would take 
3. How much does it cost on a month to month basis to run?       its costing me for my diesel , once you are set up you only have filters resin to change when needed my last set of filters have been in for around 4 months these cost £22 bag of resin was about £70 ive had same bag for over 12 months . 
Serious questions guys. I look forward to reading your experiences. Thanks.
John.