Yes Mac's can be networked together. I remember when I had a Mac IIx back in the mid 80's and my buddy and I would networks his to mine to swap software. Back then it was as simple as connecting them together via Appletalk ports. These days they are networked using ethernet or wireless like everything else.
Can they run MS Exchange?? I don't think MS has a version of Exchange for Mac OS, but Macs can run Windows, so if you did that, you could run Exchange.
Macs also can be used as servers. Mac OS X Server is specifically for that.
Can they control DVR and such??? I don't see why they couldn't as long as you had a way to connect and the software for it.
As for this "Key" thing, I have no idea what you're talking about, so I can't answer to it.
BTW, I haven't owned a Mac since the mid '90s, but know a few that use them. They're all graphic designers or artists. I'm thinking about getting one because I love the way Macs display graphics, but I don't want for fork over additional cash for software that I already have for PC.
- Dae