Just for the sake of clarity, if you purchase ProTools, you'll need to use RTAS or AAX plugins, not VST. I'm sure this was a typo, but wanted to make sure it didn't confuse.
For post-related plugins, I recommend McDSP's ML4000. It's amazingly versatile and can be used for everything from noise gating, to noise reduction, to positive expansion. Once you get your head around it, you'll wonder how you ever got along without it. And +1 on Altiverb and Speakerphone 2 (although if on a budget there are cheaper alternatives). Check out McDSP's Futzbox; it comes on sale once in a while. Not as full featured as Speakerphone 2, but good nonetheless.
I also recommend ProTools, especially if doing post in the U.S. However, as others have mentioned, if you're doing projects that don't require a tremendous amount of interaction with other ProTools users, then I say use whatever system you're comfortable with.
If you're interested in doing surround on a budget, then honestly Audition is hard to beat. Reaper is fully capable of doing surround, but it requires a pretty deep understanding of routing and setting up your channels properly. ProTools and Nuendo will both do surround VERY capably, however, ProTools requires the Complete Production Toolkit which will bring your total cost upwards of $2500 at retail. Nuendo isn't cheap either at $1800, but comes out of the box, ready to mix surround.
Good luck!
Chris Conlee