wks-
Ready to take the plunge, are we? If you have been into Macs since the dawn of time then I think you can handle this. Knowledge of networking would be extremely useful.
Apple does sponsor classes of varying lengths to customer's of all levels.
This link has more information about training options.
As for your specific reasons and my thoughts:
1. File sharing can do. The size of your files and the speed in which you need to shove the data around on the disks will determine what you need for storage options as in XRaid with Fiber card in addition to current and projected short-term needs.
2. I am very against doing it this way, although you could. I always suggest a separate box or device to run the firewall, but at the fastest speed possible. Some old G4's do the trick, as would a FireWall appliance with at minimum gig-Ethernet speed.
3. Can do.
4. Now we are getting the hang of it.
As for your questions:
1. Yes you need a server. Based on your needs, a Mac Pro could even do the trick. You do lose the option of redundant power supplies should you go this route.
2. Yes a server neophyte can do it. Depending on what you plan to administer it shouldn't take very much time per week at all to maintain a well functioning server. You just have it email you when something is up, or take a peek via Server Admin whenever you get curious. Initial setup will take WAY longer than you think, so I would say plan on two full weeks to get things going and hope for better.
3. Redundant everything is always a good idea. The option of redundant power supplies on a server is a good one to take advantage of. Buy at least one extra drive module and run your boot/applications drive mirrored-again the redundancy thing. You can get by on 2GB, but for shoving around files and managing everything else you want to do 4GB would be better.
Hope this helps.
Luck-
-DaddyPaycheck