When I started I already had a successful window cleaning round, pressure washing was an add on.
The round was sold to concentrate on commercial pressure washing - it's given me 12 good years but this is not the norm, definitely right place at the right time.
Go with window cleaning - that'll give you a solid income, then if you still have the inclination look at it as an add on or separate business.
If you DIY a WFP set up you'll do it for less than a grand and there's lots of systems on ebay.
Get knocking the doors and you'll have an income quicker than you'll develop a pressure washing business, once the round is established you'll have a customer base to sell pressure washing services.
Where are you based?