AmazingPorts Access Controller has been successfully installed on many different hardware platforms.
There are many different aspects to consider when selecting hardware, we believe that re-using existing hardware is always better than getting new if not for any other reason than limiting the negative impact on the environment.
We have successfully installed on Small Form Factor (SFF) machines from HP, Dell, Fijutsu, Siemens and IBM. These can usually be obtained second hand for 30 - 100 EUR depending on the number of machines you purchase and the configuration of the machines.
Processor: preferably Pentium 2 and faster.
Memory: preferably more than 128Mb
HDD: IDE 5 - 10 GB
NIC: most network adapters will work, but if you can choose, choose one based on the RTL 8139 chipset (most manufacturers have at least one card based on this chipset).
If your motherboard has SATA controllers, make sure they are not conneted and turned off in BIOS.