I'd go for the 28ft as it's not a lot extra and it covers 95% of all work.
Also, 1.9ltrs of resin, if DI only, ain't going to last no time at all before needing refilling which will be a pain in the rear. I'd go for the 4.6ltr.
Hose, me, I'd get the 30m.
The idea of using rain water is a good one as the TDS will be a lot, lot lower than tap water. All you'd need is a cheapo Ebay pump at around £20. Ebay - 280955194939.
Brush, I agree, get the cheapest.