With the price of diesel it still amazes me that people are obsessed with humping water about. I have a 600ltr ibc but I only use it when I clean windows, conservatorys or the very rare pressure wash job where they have no water. I'd say 99% of all other drive work I just fill the wheely bin when I get there with the customers water - even if you work with the tank in the van I'd still fill when I got there, I only use a 15l/min machine then most hoses keep up. As for the van I got an ex Balfor Beaty van for a grand with £235 in signs which is perfect - it gets there and the most important part of any van for me is the hand brake because it spends most of its time parked ! I bought a trailer bowser and have never used it untill this week when I may be forced to.
I spend the money on the gear inside and very little on the van or advertising but if your doing a very good job then word of mouth works big time. A good website is well worth the fiver a month it costs then you only need to add good content, the photos (even if their not your own) and then work on the keywords.
As for keeping busy in winter its just a case of being able to say yes when the phone rings - having a broad knowledge of the trade and being prepared to give things a go - I clean carpets, hard floors, drives, windows, gutters, jet drains and do cctv surveys, remove grafitti, cut the grass at the probation office and have a bouncy castle business !
So yes I think you should spend carefully but on the right things
"GEAR BEFORE BEER"