I have a D5000. Its wonderful, I would always go for the the 5*** range over the 3*** range as the auto focus is better. That is of course assuming that you can afford it. I would also get it body only and the the lens that woof82 recomended, 35mm 1.8 is great. I use it over my 50mm 1.4 because the range is better. Nikon and Cannon cameras tend to have more lens options from what I've been told. And I have bought 4 of my 6 lenses second hand, scoring massive bargains. Trick is to wait for the lens you want at the price you want. If you are buying, make sure you get the D5100 over the D5000, because it has autofocus while shooting video. That should be around tehe £700 mark. Key point. GO TO THE SHOP AND PLAY AROUND! or borrow your friends for a day etc etc.
Frame rate on the 5DIII wont be any different then the MKII if they stick to a similar sensor or bigger to compete with the D800 & it'll still be full frame which wont help your tele lenses. Crop also wont help tracking cars it'll just bring them a little bit closer to you (give you more reach on your teles and all other lenses). Your best bet is grabbing a 1DMKIIn or a 1DIII both will do you well in all areas.
You don't need more FPS or more buffer. You need faster and more accurate focusing (one awesome frame per second in focus over 12 frames per second with none really thought out and in focus). Remember, there is no body one the market right now that can AF and meter over 8-9fps, everything over that is trap focused and pre metered, which the 5D can also do. And the crop factor has zero to do with shooting cars moving fast, behind a fence or not. The fault in the 5DMkI/II isn't in the FPS/buffer/file size/35mm sensor, it lies squarely in the AF and metering. Canon purposely has neutered both to keep sales up in the 1D category. One of the great things that Nikon has done is to utilize the same pro level AF and metering in the top 4 pro bodies, letting you pick and choose according to need and budget.
I'm hoping to pick up my first SLR tomorrow and I've chosen the SLT-a35 as its seemed to give the most bang per buck in the sub-500 bracket. I know that Canon and Nikon have more lenses available, but I don't really think that would make too much of a difference to me in the foreseeable future I managed to pick up an old minolta 50mm f1.7 for about a tenner from ebay and got a brand new tamron 70-300 f4.5-5.6 macro for £90! and with the 18-55 kit lens, I should be sorted for a few years
Buying old lenses is an excellent way of expanding your glass - and a lot of the old stuff is pretty damn good as well. My kit has been through some changes: - EOS ID Mark IV - EOS 5D Mark II (x2) - EOS 1000D I've already agreed with my dealer to replace the Mark IV and one of the 5D Mark IIs with a pair of 1D Mark X's on launch. This means my two workhorses will be identical, full frame cameras, with my remaining 5DII for when I need that extra bit of resolution or simply as a smaller backup body. I got fed up of waiting for the 5DIII, and I've always found the 1-series AF to be far superior. Still got an EOS 1N RS for film shooting that blows the 5DII out the water for AF, despite being older and still a 5-point system. That said, I may upgrade to a Mark III when they launch if they're any good for studio work - the 1DX's downside is the resolution on the sensor. The 1000D is my backup, other half's camera and also what I'll take on holiday - admittedly, it looks stupid with a 50 1.2L on the front, but produces good images. I've also got an NEX-5 kicking around in my bag.