Yes the box was recomended for trialers or where it was exposed, the board for the inside of vans. They are both the same thing except that the board is cheaper and easier to work on. (there is an inlet filter you are supposed to unscrew and clean every now and again.)
When I was in Jonahs shoes i could't figure it out either. I rang mike at cleantech and he told me what I needed. I paid, and have always been happy with what he gave me.
What did he sell me? 400l tank ,pump box, resin vessel, 300gpd ro man Ro inline tds,, , 100m minibore with reel and tap on end quick release connections. hand held tds, pole, later I had to buy a booster pump.
It was about £1,100 I think and I had to ask my wifes permission. She thought it was a lot of money, and as I only had about twenty customers and had already spent over £300n on trad gear I suppose it was.
I don't know how long you've been trad Jonah but I found it very hard work. On my best day I earned about £70, but i was learning. I honestly believe that if I was trad now I could probably earn £100 most days, but I wouldn't be much over that. I found it daunting mentaly too, knowing i'd got to go out lugging heavy ladders about for a pittance.