Tyrol, stay away from using Windows 98SE(or 95 or ME) for any of your computers. It's just too unstable for business use.
You could use Windows 2000 Professional or XP Professional for your server. They can act as a server for up to 10 other computers. They would both do a good job as a simple file/print server for your small network. If you choose Professional for your server, I'd pick XP for all of your computers so you'd have the same look and feel on all of your computers. Going with XP also puts you on the top of the Microsoft technology curve. You can expect XP to be support by Microsoft and hardware manufacturers a bit longer than 2000 since it's newer.
If you don't mind spending the extra money, Windows 2000 Server 10 User would be a better choice. When you have a Server, your network can be a Domain. Without a Server, your nework is a Workgroup. In a Workgroup, each Win2000/XP machine has its own user account list. In a Domain, the Server(s) maintain the user account list for the whole network. Having a Domain makes user account maintainance easier.
A Windows 2000 Server can also provide a DHCP service to the other computers. That means it can automatically configure the TCP/IP protocol for each client machine when they start up. If there is no DHCP server on the network, you have to manually assign IP addresses to each machine when you set them up, making sure you don't use the same IP address on two computers.
A Server can also do let your setup fault-tolerant RAID with your hard drives. For example, if your Server computer has 2 hard drives, you can setup RAID 1. That means one hard drive is always an exact copy of the other. If the primary drive fails, 2000 Server switches to the other drive without loss of data or interruption. You can't do fault-tolerant RAID with 2000 or XP Professonal.