Here is a complete shopping list of all the bits n bobs and major parts you will need.
Tank
Something to secure it down with.
Tank outlet with a tap on it and a half inch barbed fitting. I think the 350 has a 1 inch threaded outlet.
Pump.
Pump strainer (essential)
Barbed fitting for pump inlet and outlet.
Electronic flow controller (Some may disagree)
Batttery .. 85 Amp hours upwards. Deep cycle type. Charger for battery
Metal reel.
100 meters hose (I would go for minibore).. 8mm
Tap to fit end of hose ..8mm barbed fitting to go into hose/tap and a Rectus 21 male on other end of tap.
Pole... The 24 ft H-Extel is whippy at full extension but probably the best in your budget range with a Vikan rectangular brush and a Rectus 21 Female 5mm hosetail on the pole hose. These are stop fittings so you dont get water running back down the pole hose when not plugged in.
300 GPD RO system and a DI
TDS meter
(Produce your water through RO and DI into a static tank) You won't contaminate it transferring it.
About 5 meters of the clear reinforced 1/2 inch hose.
3 x jubilee clips
1 x roll PTFE tape.
3 x H/lok STOP type fittings. (Dont worry they work fine) You will have to replace the one on the end of the hose from your pump to your reel periodically.
Sub pump with transfer hose with a trigger or tap on the end of the hose.
I reckon you might JUST scrape into the £1k budget.
The only other thing you might need is a booster pump for your RO.
I would go to one supplier to get the lot. It will save you shed loads of time shopping around and a whole lot in delivery charges. Also a supplier would look at the whole deal and give you a decent price based on the bulk order.
If you need any help feel free to call me. I am not supplying DIY kits and don't intend to do so so don't worry I wont try to sell you anything
Andrew