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If you have limited needs for a network, say for example, you just want to be able to share a printer with all of your machines, then you may be ok with peer-to-peer. It has the advantages of being simple, easy to set up (or no set up at all in your case, since you already have a peer-to-peer network), and you don't need to buy any additional hardware. Beyond that, the advantages fall with having a dedicated server.
The expense isn't that much more, since there is usually no need to have a cutting edge machine for a server. One of your existing machines would probably be sufficient. It will be more reliable and faster, and you won't have that Arrrrrgh! moment when someone shuts down a computer while you are accessing files on their hard drive. It will take a little planning and work to create a network, but not too much, and the benefits will be felt by everyone.