Do you have a specific reason to run Server 2000? If not has there been any discussion of upgrading past this? It really needs sorting.
With regard to your points;
1) You can do this with a NAS drive and running NTBACKUP from the server and storing the backup on the NAS. If it dies you can then restore to this backup, ASSUMING that it isn't a hardware failure, and assuming you can find a replacement part for a 13 year old computer <<< Is this is realistic? If not plan to upgrade.
2) You need a "redundant" server (not a backup one, that term would be misleading in this scenario, a "backup server" would imply a server that holds your backups). Since you cannot add a 2008 server as a DC to a 2000 functional level domain you need to add a 2003 server in first, do you have access to a 2003 license?
If not I'd argue you may be better looking at buying a 2012 server and starting from scratch, build a new domain on there then see if you can either migrate your existing users, or just recreate them, 40 ain't too bad, depending on how complex your folder structure is for reassigning the permissions across the network.