Putting a 75-300 on a 5D would be a foolhardy investment. It is one of the worst lenses in Canon's line up, and would perform poorly on ANY camera, much less one that demands good glass to take full advantage of the camera. You'd be much better off even with a 50/1.8 on a 5D.
What do you plan to shoot? The two cameras you list are wildly different.
If you're after Image Quality ... then the 5D beats the first generation 1D hands down. Not to be rude ... and in all seriousness ... if you actually think that these cameras are in the same league ... then perhaps you're bitting off more than you can chew ... to be honest ... if you are coming in from film and if you are proficient with film then possibly the 5D or one of the XXD .... say a 40D or 50D. If you are moving up from a P&S into a dSLR then an entry level camera like a XSi or XS.
Canon has made significant improvements in processor and sensors since the original 1D cameras all of which will have a positive affect on the IQ.
Hmmm, I'd stay away from consumer lenses...And as far as those bodies go...
What exactly do you want to shoot?
Do you want high image quality and high ISO performance?
Or do you want better auto focus and higher FPS?
I love my 5D for what it's worth and it takes amazing pictures. The downside having full frame is it's unforgiving with bad lenses yet it performs better than 1.6x cameras.
I'd go with the 5D. It may not focus quickly but it just makes pictures pop.