The above [now below as this is the 'accepted' answer] instructions will install Snow Leopard - but not technically a clean install.
To do so follow the above steps but enter Disk Utility after choosing the language in the installer. You can wipe it from there.
Stolen from Macs.com
the Snow Leopard installer will ask which language you want to use as the main language. Make your selection and click the right arrow key.
The Install Mac OS X screen will display. Click the ‘Utilities’ button.
In the Apple menu bar, select ‘Disk Utilities’ from the Utilities menu.
Disk Utilities will launch. Select one of the following instructions, depending on what you wish to do.
Erase a Volume. Use these instructions to erase an entire volume. Be sure to back up all of your data first.
Format a Hard Drive. Use these instructions if you wish to erase an entire hard drive, including any volumes/partitions it may contain, and not create any new volumes/partitions. Be sure to back up all of your data first.
Partition a Hard Drive. Use these instructions if you wish to erase and partition a hard drive. Be sure to back up all of your data first.
When you have finished using Disk Utility, select ‘Quit’ from the Disk Utility menu.
You will be returned to the Snow Leopard Installer to continue the installation.
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!
Best Answer
No it would not work. You will probably get a segmentation fault due to major changes that happened between 10.6 and 10.8 (I don't think you could even make 10.8 binaries work on 10.7). You can easily try it, however, to convince yourself. Extract them with Pacifist to some temp directory and just run them (eg. extract ntpd and do a
./ntpd --version). If you don't get a segmentation fault and you get a printout of the version, you might be fine. However, there's a better option.Instead of using Apple's binaries, you should compile NTP yourself. It's not difficult at all and it requires Xcode (which is free anyway).
See my answer to a similar question that was posted the other day on how to compile NTP: Disabling NTP on OS X Lion or older