I don't see any issues with purchasing a 5d MkII. It is a lot of camera and as has been mentioned, your current lens set will not really take advantage of this cameras full size sensor and amazing 21mp ability...
The 50mm 1.8 is a fine lens, but not a top-line lens and the 75-300 is usually considered pretty soft, especially at the long end........
A camera like this really needs Canons top-end "L" glass.. If you have been shooting with a crop camera you also need to be aware that a full frame camera will give you a different "look" because it will appear like your lens is 60% shorter than before...
Therefore, 24mm is a good wide angle where you may have needed 17mm on the crop camera for the same look....
I would now say, how much do you want to spend...
The 28-135mm Gold-ring lens is pretty nice and frequently seen with the 50d etc. A great lens for a reasonable price...
It will cost you triple for the 24-105mm-L, which frequently came on the 5d
A heavy, but spectacular and pricey lens, would be the 70-200 f2.8L, or f4L if you're on a budget...