Hi John, welcome to the forum.
There are many different types of set up; but the main ones I can think of are:
Van mounted
Trolley System
Backpack System (often mounted on a trolley)
and yes, you can build a DIY version of all three.
Matt, a site member has created his own DIY WFP site here:
http://d.co.uk/The above link is an excellent place to start as it gives you a 'walk through, talk through' lesson in what comprises a WFP set up; WITH PICTURES; which greatly helped my own personal understanding.
It also has a forum, so you can ask questions and Matt's a helpful guy too.
But to answer you questions:
...and would imagine that I would need a tank in the back of the vehicle carrying ro water that has also passed through a DI unit to the various jobs.
Yes you can install a tank in your vehicle, but it's not always necessary, depending on what system you use.
is wfp the way to go, can I build it myself, from where is best and why?
Judging by the WFP members, yes it's the way to go and yes you can build it yourself. See Matt's site. If you use the forum there, don't forget to log in and introduce yourself.
It's only polite.