If it's just small office files, it will suffice your needs. For comparison: I have a small Pentium 1 200 in my parents' office doing all that. Your machine is much faster than this methusalem.
The question about the OS: Yes, Panther can do everything just fine. It depends on how much Unix you want to learn/you already know. Don't worry, with webmin, you can do almost everything with a GUI, if you wish.
If you insist on Apple's GUI tools, then you can still buy OS X Server.
As for the remote access, you could use sftp, there are some GUIs out there. Basically, it is similar to ftp, but everything is encrypted.