My tuppenceworth, for what it’s worth:
Firstly, I don’t agree that you need to know how to clean windows the traditional way. I would forget that and focus only on water fed pole. If a job comes up that requires traditional (extremely rare these days) just decline it.
Secondly, and this may be a bit against the grain, but I would NOT mess about trying to cobble together a system for cheap. Sure it would be a steep learning curve, but I did something similar, and my conclusion is that I would have saved myself a lot of time, trouble and even money by just buying a ready-made professional system that worked perfectly straight away. (This is what I now have)
I don’t even think it would cost more, especially when you factor in your own time.
My own system is from Pure2o, which is part of Ionics, and really top quality. I think it cost £3,700 including fitting (700 litre). They also have reconditioned ones for sale which are cheaper still.
I think a single operator would need at least a 400 litre tank, so that means a small-to-medium van (such as vw caddy or similar).
Lastly, I would never buy a van and system like this outright with cash. Finance it over 3 or 5 years via a personal loan or asset finance company like Kennet.
Good luck!