WOW!!! LOTS of issues. First off, do lots of planning. And go over this planning scheme like 20 million times. I'm assuming you'll be looking at going to Active Directory, so alos make sure you can afford all the Client Access Licenses (CALs). So I'll try to chime in on some of your questions....
1. Yes, your hardware should be good enough, better run through the HCL to make sure it works. I'd suggest going out and spending some money on a new one, maybe work the "time is money" angle, and how disk drives are huge now, so your marketing guys would be happy with bigger drives...
2.What kind of firewall is up to you, suggest going with a seperate piece of equipement so it's OS independent. Maybe a Cisco Pix??
3. Mirroring software? If you are implementing a RAID system, hardware RAID is faster and more reliable. But if you want to use software RAID, then W2K has some built-in (as does XP).
4. W2K servers are solid and reliable now, after you get in all the updates (just don't install IIS). As for XP... it's not released yet, but will further push the W2K envelope, so it should be an easier upgrade.
5. Network speed? hmm not too sure there. This usually depends on the type of wiring, any crosstalk on the lines, the amount and type of data flowing through it, as well as security issues and the network cards in the machines. LOTS of variables to look through. Overall, I doubt it could make it that much worse....
6. Remote management with W2K is fairly easy. Just instal the Terminal Service in Remote Admin mode. This will allow you, with the correct security settings to access the servers from anywhere in the world using a terminal services client (one on the W2K CD, or the built-in XP). This will again depend on the connections and permissions.....
7. yes, to move from Novell to Microsoft is basically like building a network from scratch, except that you've already got the hardware. LOTS of money in CALs and server OSs. So make sure you've got EVERYTHING planned and budgeted. Maybe it might work for you to buy a newer server and begin slowly shifting people to a W2K network.
Anyway, lots of luck and remember to think things through.....