I suggest buying Used, and getting better glass that will definitely be more future proof. If you are going to pay retail for a 17-85mm, you may as well pay a bit more for a used higher quality glass, like a 17-55mm f/2.8 IS or an L lens, especially if futureproof is what you are aiming at.
It all depends on what you shoot also, what would be more worth investing in for your situation. Since you were considering a 10-20mm, I assume landscapes are one of your interests? If so, maybe getting that lens over anything else would be more suitable if you will get the most use out of it.
Either way, I don't think the 17-85mm is that future proof as I've read it isn't that great. Nifty fifty is a no brainer since it is so cheap.
If you were considering getting all you listed brand new, you could probably pick up in the used market, 40D + 17-55mm f/2.8 + 10-20mm + 50mm f1.8. Not massive amount of difference between 40D and 50D, you will get more from investing more in lenses.