The 15-85 is great for landscapes. The EF-S lenses (15-85, 17-55) don't work on FF. Personally, I recommend buying the lenses you need for the camera you have now, not a camera you might get at some unspecified future time. The high-quality EF-S lenses that I listed, along with the 10-22 ultra wide, hold their value very well. More importantly, outside of those lenses that I listed (and some 3rd party counterparts), There really is no high quality general purpose zoom lens (wide angle to short tele) for both APS-C and FF.
if you buy them used or refurbished you could sell them for about what you paid. if you buy new, you are looking at about a twenty to thirty percent loss when you sell them. But the same can be said about most lenses.