A couple of things pal. I was in a very similar possition to you when I was 18 and think you are on the right track, you can make more than just a good living with window cleaning, but here is where I think your inexpierance might hinder you in the long run.
Buy a better van, pay it up if you have to, and dont just quit your job. Start window cleaning now, and window clean when your are not in the supermarket. This will give you a head start.
The reason I say this, is that you really want about 400 customers, but it takes time to accuire them, and accuire good ones at that. When you are drinking from a bottle of the barrel you get all the crap, that everyone else left, and you will find yourself wanting to do allot of refining.
Also as a few have said, learn trad methods, which will take you a while to get used to, but if you only do outsides, you'll find you dont get allot of, one larger jobs ad two commericial contracts, as they tend to want both inside and out done, so if you cant swivvel, you might find it difficult.
£100 marketing is way of as well, my local newpaper charges about that for one add on there front page. You should get out and start knocking doors right away, you can start making up flyers, brochures and poster etc now, when you are still in the planning stage.
Hope that all helps
Gerard
