1 x 650 litre tank = £220
2 x hose and reels = £200
2 x pumps = £170 (ish)
2 x flo controllers = £130 (ish)
1 x can protectacote = £65 (ish)
Some other bits and bobs = £30 (ish)
Total cost for system = £617 (ish) plus the cost of RO/DI, holding tank, and transfer pump (I don't know how much ROs cost these days).
You could spend £3.5K, but at the end of the day, it's just pure water coming out of a brush head, and the money you save you could spend on top notch poles and a better van.