Well, from reading this it looks like the set up either a software or an cheap desktop computer class raid setup on a pair of IDE drives. I wouldn't even recomend running something like that for a home user. They brought out two 250s because the 200s probably weren't available anymore and it was all they could get.
My first recomendation would be to buy a real server with a hardware raid controller running either scsi or sas drives. I just bought a pair of off lease HP ML370 G3s for two side customers. They were around $500 US and had 6 72.8 gig Ultra 320s, redundant colling, and redundant power supplies. If a drive fails you should replace it with a matching drive, most real servers even prefer you to hot swap them (don't try that with your box though). As for your DC, that box is probably dead. Microsoft has a step by step document for moving all the DC roles to another server.
How to replace single domain controller in domain with a single domain controller? As for DHCP, I moved all my dhcp to my switches and took it away from MSFT. I had problems with clients moving between VLANs and getting assigned the address for the vlan they moved from and not the vlan they were on. But you will probably need to do an ipconfig /release and ipconfig /renew to get them to pick up the new DHCP server.
