Sounds like your on a budget.
I wouldn't get a pump box if I were you.
IMO they are the biggest rip off in the world of window cleaning
Whilst I agree that they are expensive for what they are, they are for WFP what plug and play is for Windows (media). Everything is taken out of the van in one package for charging and frost protection in the winter.
I'm not saying having everything in one box isn't a good idea.
I know what the box its self is worth and how little effort goes into fitting a pump and controller into it.
The mark up for this is astronomical.
I've got my pump mounted on a nice thick piece of oak that I had left over from my kitchen work tops which is fixed to the tank
There's space on there should I wish to add a controller in the future.
It can be uncoupled and brought inside with ease and I can
see everything.
Took me half an hour make this, mount the pump and put it in the van.
I bet I could make a pump box up even quicker.