Install Snow Leopard Server
Unfortunately, the disks that come with a Mac are Mac-specfic. They have special drivers for the machine they came with, and usually don't include drivers for other machines. The serial number he's providing you with should still work with no issues, but the disks themselves will probably not work. I would suggest getting the Snow Leopard Server disks from another source and using the serial number you're buying. 

As for installing Server on top of the existing OS, this shouldn't be an issue. I would still recommend having a backup, but it shouldn't erase any data.

Good luck!