1. yes, Win 2K can do ICS, see
Here.
2. Any one you like.
3. Some IPs will only accept one MAC address, that's the advantage of using a router, it gives it's MAC address and you can hang up to 254 PCs on in the back of it, I don't think you'll exceed that...
4. VPN is a matter between each PC, the router and the internet, additional PCs connected to the router are not a concern. Just make sure you buy a suitable router with a multiple VPN capability. e.g. see
here.