The kit lens is excellent, and you will love it.Are you talking about the 100-300mm f/4.5-5.6 USM? Or the 100-300mm f/5.6 or 5.6L? I have the former and find the image quality at 300mm to be quite poor. The focus speed, build quality and IQ at <200mm are quite decent for the price, but I think the IQ of this lens is the worst of all the Canon xxx-300mm zooms they have made. I have considered selling it to buy the 70-210mm f/3.5-4.5, 70-210mm f/4, 100-300mm f/5.6, or any of the 75-300mm lenses.
Once you get a FF, focal lengths are suddenly more limited, so you might be looking for longer focal lengths soon.
Fortunately, there is a good assortment of older Canon telephoto lenses that perform reasonably well on the 5D MK II, so don't be afraid to pay $100 for a old 70-210mm f/4 zoom, its a old push-pull design, but will fill a gap until you can afford a top of the line lens. I gave mine to my daughter and she loves it. Another one, a 100-300mm lens can often be found for $100, and its respectable even though not spectacular. I have had several of both, that came with old film bodies bought on Craigslist.
I also just picked up the Tamron 200-400mm f/5.6 for $100, and while the image quality is much better than the 100-300mm USM, the focus speed is nearly unusable, so I might just list it on eBay and make a little profit from the flip (they normally go for $175-250).