All the 3x20 (3000) series John Deere tractors have the same frame size, the only difference is power, hydraulics, etc. So if a 3320 would work a 3720 would be a 3320 with more power but no difference in physical size, if that makes sense, they have the same footprint. You won't see a larger frame until you go to the 4000 series tractor.
With that said I think you would be happiest with either a 3520 or 3720. If you need a lot of PTO work the 3720 is the way to go, but if you don't the 3520 will work. Once you get to the 3720 you will not notice anymore pure power as you will run out of traction before you do power. If you will be doing more than just a MMM (mid mount mower) as any 3000 will run one a 3720 may not be a bad idea, and I've never heard anyone complaining they have "too much" power.
As for the loader, go with the CX. For a couple hundred dollars you get a larger lift and a quick attach/detach loader.
I would recommend a box blade as well for a nice ballast and is great for moving dirt. A ballast box is great as well, ballast is necessary to safely operate your tractor.
Another thing to think about is a cab. They may add a lot to the base cost of a tractor but the comfort is nice over a lifetime. Nothing beats being extremely cold or hot outside and being nice and warm inside.