good advice here, although i'd look at selling the 15-85 too and picking up an 85mm f1.8 too look i would worry about a teleconverter just yet but when you do look at the kenko 1.4 300 dgx great value for moneyNot skimping on the lens is exactly how I ended up with the 70-200 2.8 IS Mk2. Easily the best advice I've ever read. It's been amazing and I wish I would have picked it up sooner. The 24-105 and 24-70 are two that are for sure on my short list should a body upgrade not offer much.
In terms of selling, my only problem there seems to be the market. I've tried to ditch the two kit lenses (17-55) twice with no real luck. Best offer I got for one was $35. At that price, I'd rather keep them around as emergency backups. The 55-250 doesn't seem like a real hot item on eBay or other forums with "For Sale" sections, but I could give it a go. The real winner seems to be the 15-85 as it looks like I could unload that fast (even though I do love it) for close to what I paid for it last year.
All great advice so far. I'm already looking into some fast, wide 2xmm lenses that I never would have thought about. Please keep the ideas coming!
I'd suggest skipping the midrange zooms for now, your budget is limited and your 50mm has good value. I would suggest upgrading to the 5dii and selling your 55-200 and 18-55s for the 17-40L. That would give you 17-40, 50 and 70-200, so you really do not need a midrange zoom. That would give you significantly more range on the wide end (17 vs. 15*1.6), which is useful. You can then get the 1.4x TC to restore the range on the long end later if you need it. So, for slightly more than a net of 2k, you'd have a large range (17-280 with the TC) and you would have moved to FF.