Upload & Sell: On
Another way to look at it is this:
Maybe the 5D has depreciated so far that over the summer months it's not going to depreciate in absolute terms as much as the 5D2 is likely to depreciate. (By the way, I don't know if this is the case, but as a potential buyer you'd be wise to look into it.)
That would mean that buying a 5D now, since that's all you can afford, will let you shoot with the camera all summer. Then you can sell it for a small loss ($50/$100) and buy a 5D2. You'll either lose $50/$100 if the 5D2 used prices don't change (I don't expect them to go up, the 6D is putting downward pressure there), break even, or even make a profit when considering the opportunity costs (say, if the used price of a 5D2 drops more, in absolute terms, than the used value of the 5D you go to sell).
The major caveats are if you lose/break the 5D, have to put any costs in if a shutter fails, etc. Though those are probably safe risks to take on if you only plan to keep it for 3 to 5 months.
You can have your cake and eat it too, it seems. Both cameras are a stunning value.
As for IQ/low light, the 5D outclasses the 7d completely. The 7D is a fine camera, but the 5D is simply better when it comes to IQ. The 7D wins in AF. The 5D2 has all the advantages, but even more: better low light as a result of MP + sensor/electronics/processing improvements, video, AF MA, LCD. Autofocus using centerpoint is fine for the vast majority of folks, even those shooting weddings, kids, and such. I wouldn't use a 5D2 for sports; but I would use a 5d3.