What really should be thinking about is an outboard hard drive or 2 - heck of a lot cheaper than buying another computer and gives you a backup of your stuff.
I always buy combo firewire/usb outboard hard drives so that I can plug the thing into anything - like some of out old iMacs and iBooks without firewire ports. You use the firewire when yoo can because of the transfer speed. You just plug it in and it mounts to the desktop.
These things are not expensive and are availabe at places like CompUSA. One mac specialist with good prices and service is
2 of them will cost less than the mini.
If you do want to transfer to another machine, it's easy. You can use a crossover cable to connect directly between 2 machines, but it's usually easier to just get a basic ethernet switch and basic cat 5 cables.
File sharing is easy but it's different on Classic and osx - the Public folder in osx adnt he Shared folder in classic. You did not say which one you use. You can a just go to the help menu and look at file sharing.
If you are loking at a lower priced mac, you might consider the $799 eMac instead of the mini. It's a better machine, with a far faster hard drive and includes a good monitor. It's far easeier to upgrade memory than on the mini. To make it work better, I'd recommend adding 512 mergs of ram. If you go to Apple's online store, you might find refurbs, with same waranty as new macs, at a saxcings of $100 or so,