This question is likely to get ripped to shreds...but, do you mean a standard PC with Windows 7 from your local Supermarket, or a actual proper server with Windows 7 installed on it?
If you are just buying a PC to run this, forget it. A PC, even a powerful one, is nowhere capable of running with the fault-tolerance that a server would be capable of. A PC would have one PSU, and one network port, so if either of those die then that's your server gone. It's also unlikely to have ECC RAM so if errors creep into your data and that's your data intergrity gone. Finally, it's only going to have one hard disk in, so if the disk dies...that's everything gone. Another one on the disk is that it's going to be slower than disks in a server (5,400RPM/7,000RPM compared to 10,000 RPM for (some) server drives).
Yes, servers are expensive to acquire and set up but it is worth it. Really.
As for installing Windows 7 on a server, well, it's doable, but I wouldn't really want to as Windows 7 has a much higher overhead than a Server 2008. Either way, you'll have to get 64-bit (unless you're going to run a DB with less than 4GB RAM...could be interesting).
There's also the Licencing, I don't even know if you are allowed to use Windows 7 as a server. I know in XP there is a limit on connections to the machine, so you may also fall down on that.
EDIT: as noocyte has said and linked to above, the limit on connections is probably higher than XP, but definately NOT enough for 300 users.
TL;DR Version: Pony up and get what you need. Don't skimp on it cause you'll regret it later.