James,
If you haven't a family depending on you, go for it. Take the shop job and go 'balls out' to get enough window cleaning customers to make it work for you.
You've got the 'hands on' experience; you're probably more experienced than many of us, since you've worked at 60 feet. You know what Winters are like; you're going into this thing with your eyes wide open (apart from knowing what your cash flow will be like). And so what if you end up eating Aldis baked beans for a few months on end?
But if you've got a family who rely on you to feed them, then you've got to be more conservative.