Here is what you need if you just need a Server-Client Environment:
1) Ubuntu Server
- Recommended System Requirements
- 1GHz Pentium 4 should be fine (no need of fancy hardware, Linux works best on old hardware)
- 256MB RAM + 1 GB HDD (without GUI)
- 512MB RAM + 5 GB HDD (with GUI)
- *GUI - Graphic User Interface or Desktop
- (More information available at Wikipedia and Ubuntu Documentation)
2) Wireless Router
You can connect the WiFi Router to the ADSL/Dial-up Modem for internet access (if required). Enable DHCP on the router and have all computers and the Server use the same network.
3) Build the Ubuntu Server using this guide
Install LAMP and other such services required and configure them to be accessable via the local network.
For example if your Main Server and Local computers are on the
192.168.0.0/24 network then you should be able to access these services via the browser on your local system by simply typing in the
Server IP address into the browser.
It might be good to look up SSH Server, Apache Server, Samba/File Share Server, OpenSSL & Print Server in the Ubuntu Documentation for other services - available in the server guide. It would be wise to look up Setting up a Firewall or Security in Ubuntu if you are going to expose the server to the internet.
If you already have the Server Hardware check the HCL (Hardware Compatibility List) of Ubuntu to ensure that you meet the required specifications or you may land up spending hours looking up Drivers or trying to get your hardware perform to its max.
I hope this has addressed your question.