If you want to keep you the cost reasonable then keep it as simple as possible I set my van mount up for less than £500.
It consists of 400l upright tank, pump, car battery, 2 di vessels,
hose reel with 100m of microbore bore hose & a Harris pole which I made myself (see YouTube for instructions)
You can use a controller but I don't bother I just work fast.
A pump box is basically just a box that somebody has put a pump, battery & controller into but they charge an awful lot for doing so over and above the component parts.
If you live in a hard water area you might need an ro but you need to buy a TDS meter and see (about £15 off eBay)
I bought everything second hand except the hose and reel, the di's were reconditioned off eBay. Nothings failed yet either.
I may upgrade to a Gardiners pole at some point but the Harris is a great little pole and very light.