The kit lens is excellent, and you will love it.
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.
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.
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).
I was replying to the OP, he noted that he was on a budget. You are welcome to the Canon 75-300mm lens, if you think it is great, go for it, I'm not going to knock someone for their choice of lenses, we all have to make buying decisions that are right for us. The 100-300 is weak at 300mm, but its strong at 100-200mm, and its cheap. It and the 70-210mm f/4 which is better, both pickup where his 24-105mm L leaves off. A 200-400mm lens leaves a gap for him to fill.
Obviously, we would like to recommend top lenses, but if he is looking for a place holder to extend his focal length for now, the 70-210 would be my first recommendation.
Here is a cat photo with my 70-210mm f/4 taken a few years back with my 30D or 40D. This is a 100% crop at 200mm and wide open aperture.
Impressive shot. Have you owned any of the other lenses I mentioned? If I do decide to sell off my telephoto zooms, I would like to get the best bang/buck replacement (since they are all in the same price range) and I would value your opinion on the choices.
if the long zoom needs to cost less then i'd get 55-250 or tamron 70-300 vc over the ones you mention
Except that the 55-250mm is an EF-S lens that will not even mount on the 5D3.