Any generic X86 computer may serve as a node in pplznet. x86 is an incredibly flexible platform as it can have a size of a big tower as well as a tiny NUC kind of device.
The important part is to get decent network connectivity. Very popular NICs are Connect-X 3 cards because they are cheap and they don't draw as much power as first generation 10G cards did.
x86 is a good platform if you would like to run a Polynode [1] , since it's pretty expandable.
Good examples of x86 devices: