FWIW, I'm not sure you'd need to paint a wooden one. We've been buying pressure treated timber stuff for the garden for about a decade now. Not had any of it show any sign of rot setting in (even on flat horizontal surfaces that hold rain/snow for extended periods).
We recently moved and bought a new 10x8ft shed designed to my own spec - tanalised (I've no plans to paint it), with heavier frame/floor/walls than standard - that came in at about £1200. Shed chappie was saying the price tends to largely depends on floor area, so you'd probably be able to get one you could fit a car in for ~£2500-3000.
Base took a day to lay (ordered some sand/cement/large Local Authority type slabs) on a DIY basis.
One word of caution if you do go down the large shed route: budget a little extra to increase the security. Most sheds get supplied ready to be broken into with a cross-headed screwdriver...
Our old garage however had a wooden front to it and needed treating every couple of years. Was a bit of a hassle to do and didn't look as good as the pressure treated one.