Buy something fairly basic. (It will never up to date or big enough!)
Check they are covered on the house insurance.
Buy a couple of external hard drive - bribe the boys to do backups every week.
Keep all software disks in a safe place - supply a large biscuit tin with each computer.
Keep a note of all machine serial numbers and any sticker key numbers.
Before you give them the machines make sure the each have a good anti-virus and firewall set up to automatically update.