For a water fed pole system you will be looking at about £3000, you more than likely need traditional window cleaning equipment aswell. What sparklingwc put about doing trad first I would say the same thing to start with unless you are made of money. If you do have money to spend and do not mind the outlay go straight to wfp.
You could start off just using ladders and a squeege etc or go straight into wfp.
Pros for the trad setup is cost only a couple of hundred.
pros for wfp gets windows and frames cleaner quicker than trad and stay that way longer work can be done quicker after first couple of cleans. (cons for wfp is will take more time at first to get used to it and cost alot more than trad)
There are a few things I would be first before going to buy anything.
1, window clean your own house (that is if you have not window cleaned before)
2, go out and get some customers either leaflet/ knock doors or both. without customers you cant work so dont go out and spend money if there is no need.
If there is not a window cleaner then you should pick up quite alot of work. (remember to dress smartly"smart casual" if knocking on doors)
set a price you want to earn per hour or per window and stick to it.
The list could be alot longer but you can get info overload