Help!! Help!!

This depends completely on exactly what model Mac pro silver tower, dark cylinder, or notebook computer you own.

You cited the article about serial number. At least some of the model information or list of features should be in the same area. Please post it, or a link to the ebay listing, or anything that would tell what Mac.

To identify your Mac enter serial number here

http://www.everymac.com/ultimate-mac-lookup/

sorry to hear about your hard disk. In answer to your questions

Yes-technically. It depends if your Mac Pro meets the basic requirements for that OS, including memory on board

and

you will need a copy of the original install DVD's or Snow Leopard which is around $20. Call Apple itself, they will sell you a set of both if you ask them.

to boot from the dvd drive, reboot, hold down "C" key and load in the 1st dvd. A new hard drive is easy to replace. It has to be sata 2, and all you'll need is a screwdriver to do the job. you could start out with 1 or 2 TB and go from there. You will need to format it/partition it before you can use it and install the OS

on it

good luck

John B