'm a first time buyer of a Canon camera and I'm getting a 5D today. I'm using it for short films and music videos, etc. I have an EF 18-55mm f/3.5-5.6 and an EF 70-300mm f/4-5.6 lens. So, would it be a good idea to just buy the 5dmkii body only... or does the kit come with other stuff when you the get kit? (I also want to get a 50mm 1.8 or 1.4 lens.)
The Canon 5D Mk II has two options, the camera body by itself or the camera body and the Canon 24-105mm f/4L IS USM AF Lens. Some stores such as Adorama also package additional items with the camera such as memory cards, batteries, bags, cleaning tools, or a grip. These are not kits put together by Canon, they are just bundling additional items along with the Camera at one price point.
While considering your current lens lineup, the kit lens is a huge step up in terms of quality. The 24-105mm lens is a professional lens, and as such has many advanced features that your current lenses do not offer. Another thing to consider is that you save about $300 when you purchase this lens as part of the kit and not separately. That alone makes it a great option for many. Unlike many kit lenses, the 24-105mm is actually a very competitive option and well worth considering.
Overall, I think the 5D kit is a great option for your particular case. The Adorama "kits" are not really necessary and probably include knock off brand's that you don't need.
Unless I'm mistaken, the 18-55 is an EF-S lens, not EF, and won't mount on the full-frame 5D. Per Canon's lens lineup page, http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup, the 70-300 is an EF lens and will mount on the 5D.
I've never bought a body new, but my impression is that a kit is just a lens, usually at a small discount, along with the body; if you are going to buy the lens anyway you might as well get them together. It sounds like you might need a lens "similar" to the 18-55, and the 24-105 that @dpollitt mentioned would do the trick; the 24-70 might also be worth considering.
Also, did you consider getting a 7D (crop-sensor) body instead? The 18-55 (not that it's a terribly expensive lens) would work with that. It would be a step down in image quality from the 5D, though.
Among your existing lenses, the 18-55mm is an EF-S (for APS-C Sensors only) lens and will not mount on the 5D Mark II body. The 70-300mm will mount but its a longer lens than your interests (short films/music videos) will need. I'd suggest you get the 24-105mm f/4.0 kit lens. If money isn't a problem, get the 24-70mm f/2.8 instead, it will complement your 70-300mm lens better. Also getting a 50mm f/1.4 or f/1.8 is a good idea for doing video works.
The 5D MkII has a lot of possibilities when purchasing: you can go for the body only, standard kit or many sellers provide their own bundle deals with very good lenses. The 18-55 is an EF-S lens so will not mount on the 5D because the 5D is a full frame sensor and EF-S lenses only work with crop sensor bodies. So if you need a lens with it I would recommend you go for a kit with L glass, or the last option because a camera of this quality really requires good glass.
As you're going for a 5D MkII I'm going to guess that limited budget is not a prime factor. Given this I would recommend going for a bundle offer with a Canon L series lens, something like a 24-70 f/2.8L which is a very good, if expensive, all-around lens whose quality matches the camera body and is supplied as a bundle with this body by some sellers. On a full frame body like the 5D this will be roughly equivalent to the 18-55 you have on a crop sensor body.
If you want to use the 18-55 you have you would need to consider a crop sensor body like a 7D which is a fine body having the same build quality as the 5D MkII but more up-to-date features and especially better video capabilities than the 5D MKII. If you're going to be doing a lot of video with this I would recommend a battery grip as video drains the battery quite quickly on these cameras.