At the moment that box is a Core i5 PC box running 4 Gig ram, 500GB Hard drive and Windows 7. Is that good enough or should it be upgraded? And if yes, then to what?
Also, will I have to change the operating systems on all the pc's or can they all remain as is (Windows 7)?
That should be fine. I would however put another hard drive inside the box to mirror the existing one. I would also get a external eSata drive or something like that to act as another mirror you only hook up once a week and then store offsite (home safe etc) incase you have a break in and the box gets swiped. Sorry, I'm paranoid about data loss, even more so when it involves business data/mail as it's not something you wanna lose and the cost in hard drives is insignificant compared to the damage to your business. You might have to bump ram up a bit but I doubt it and even if you had to it's cheap. Do however get a decent APC UPS for the PC to run on seeing it's your server and that will ensure nice clean power for it taking care of spikes & brownouts that has a tendency to kill electronics.
No, all your other PC's will remain as is with Win7, no changes required in OS. Might just have to adjust a few network setting to point it to the new server. Only the box running VPN/Server will have CLearOS installed.