I cannot afford anything in the 1 series, so that leaves me with a 5DmkII or a 7d...
I keep reading that the AF on the 5 is really pathetic...
I am primarily shooting sports, but those with linear/predictable motion and a fairly big target (e.g. running races, bike races)
If you are primarily shooting sports, you need the right tools. Let's get this straight. The 5DII delivers almost unrivalled files. But it ain't a sports camera. With the budget you mentioned, if you must buy new, the 7D (with a grip) and f/2.8 or faster lenses is the way to go.
But think outside the square for a moment. You clearly need a 1-Series body. The astonishing shots run in Sports Illustrated from the 2004 Summer Olympics were most likely shot on a Canon 1DMkII. Fast forward a bit and you get to the almost legendary 1DMkIIn, a body which I still own and occasionally use. It's as responsive and quick as my MkIV.
Fast forward a little further to the MkIII. The early copies may have had some AF issues but pick yourself up a late MkIII and you have got yourself a VERY respectable sports camera.
Don't worry about a well kept older 1-Series as far as reliability goes. They are carved from a solid lump of Unobtanium. My 1DMkIIn must have over 500,000 actuations and still looks like new and purrs like it did when new. And for shooting action it will beat the pants off any 7D.