Getting customer: buy some established work as you will be getting customers who want a window cleaner and are used to paying for the service, after you are up and running that way Leaflet drop with a follow-up door knock about Five days later, then you can choose your area and quote on the spot to anyone who shows interest.
Get a sign written van,
I would strongly advise a DIY system, Tank, Battery, pump, controller, hose & reel, Pole and Brush can all be purchased a lot cheaper individually than as a package, plus you have the knowledge of how everything works.
The get a job brigade are not wrong, my missis runs a warehouse where the average wage for say fork lift operative is circa 33K a year, you won't get that kind of money window cleaning for at least two years.
All I can say is if you fancy it and are prepared to work hard do it, if it's just a whim and fanciful, go on someone's payroll.