Agree with Ian too. Plastic angle adapter( I believe is already fitted on the 17' xtel). Do not buy a goose neck.
Cleantech is a good supplier you haven't mentioned- their site is clear and Mike gives good advice. You are fairly lucky because you can trust all of the suppliers.As far as I know there are no bad ones.
Cleantech sell a pump board. This has a varistream, filter and a pump already assembled. All you need to do is connect a black wire and a red wire to a leisure battery, and this has male hoselocks both sides to take water from your tank to your reel.
Cleantech give the dimensions of tanks on their site. Go for the biggest you can fit. Consider ripping out that racking you mentioned.
As regards differing opinions over best poles, the slx is the best but at £400 it is dearer and has a longer closed lenght because it is 25' (25' divided by four). The DIy harris is the cheapest and very robust but you will have to spend time biulding it (putting a hose through it that you will have to buy seperatly and a brush on the end). How much is your time worth? How much are you saving?
One hole in our advice though is that I/we keep harping on about the closed lengh of a pole.This is because we all use them for ground floor cleaning. You mentioned you will not be doing this in which case closed lenght is not such an issue.
It's only my opinion but I think total wfp is better. (I mean do ups and downs for business and practical reasons)
I like a reel fixed inside the van because lifting it in and out at every job is wasted effort.
A harness that fits over your head (Ian Giles idea) is the best way to attach the end of the hose to yourself- you walk away and the hose comes too! I have a unger pouch on my waist belt and in this I have a scraper, green scourer, and microfibre.This has to do with speed, wfp will get any stain off, but a scraper can be quicker and a walk back to the van for a tool wastes time.
Look back at Jeff1's posts when he was planning and then switching to wfp. The difference is stark for someone who already has, as opposed to someone who is biulding a round. He had been going for nine years, and then towards the end of his first WFP fortnight he had his best earning day ever. Imagine that, try and get your head round it. You are very good at what you do, you work hard, and then you make a change and even on an easy day when you finish earlier you still wipe the floor with all of your previous best earnings. Should you be happy? Or cry at all those wasted years?
One critisism I would make of his switch is that in deciding which were the best tools, preparing his work, and informing his customers too much time was wasted. Just get on with it.