I started with a backpack as I was in the same situation as you (limited funds)
Here is what you will need...
Backpack (WCW)
Spare battery (maplins £30)
RO with booster (300GPD)
25L containers
Pole or 2 (30ft & 20ft)
IBC tank (better to have more than not enough stored)
Transfer pump
Trad equipment (for insides)
Step ladders (con roofs)
Couple of brushes (get a superlite for fascias, there brill)
TDS meter
Couple of spare goosenecks
A pouch on a belt for cloths