Welcome to the Forum! Great to hear you're stepping up to the Semi-Pro world! Congrats.
What a step up from the 350D to the 5D Mark II!! WOW!!
Remember that the 5D is full frame, so you lose the free extender built in with the crop factor of the sensor.
If it was up to me, and I was strictly shooting wildlife, I'd skip the 5D and go with the 1D Mark IIn. If you look through many of the other forums, especially POTN and Fred Miranda, which both have bird and wildlife specific forums, you'll see most of the pros are using this body. It has the 1.6 crop factor which is like a built in tele-converter, yet you lose no f/stop, and it's as fast as one could hope for at 8fps! This will really help when shooting birds, or any wildlife that is moving, as most are...LOL
It will be my next body and my 5D will become more of a landscape and studio body as I dont like to use it too much for anything that needs fast action photography.
Plus in the amount of money you'll save compared to the 5D Mark II, you'll be able to afford another lens.
As for lenses, I'd recommend the 70-200 f/2.8 IS. It's probably Canon's best zoom on the market and most compare it to a prime it's so sharp! We own that lens, and it almost never comes off one of our cameras, it's just that good! With the amount you'll save from going with the 1D, you could afford to get both the 70-200 and the Sigma 50-500! I've read many reviews of this lens, and except for the fact that it's not a 2.8, I've seen some amazing images with this lens. I believe there are a few members on this forum that own it and could maybe post some images shot with it.
As long as you're in good light, I think its a f/5.6, 500mm should get you out to where you need to be for wildlife!
Like Bob said, if I was going to be buying new stuff for a project, I might try to rent the lenses first to see what you like or dislike with them. I know the prices to rent these is fairly inexpensive and they might even put it towards the purchase of the lens if you decide to buy it. If it isnt this one, then I know there was another rental company that offered that perkhttp://www.borrowlenses.com/category/canon_super_tele
Good luck, and let us know what you decide on. I'd love to see some of your work too!