If you are not looking for more than 4 frames per second, don’t mind relatively slow auto focus and are content with ISO up to 2000 then the MkII is a bargain. I shoot birds in flight and manage with about 80% in focus. I bought my MkII secondhand 3 years ago. The MkIII and the 1Dx offer faster focus and processing rates but not much more.
Spend your money on a good lens for your second hand MkII. Check the shutter count, they are designed for 150,000 shots. You may need to spend $300 on a clean and o/haul.