Compile your own kernel for Plume workstations
You'll need at least a 2.4 Linux kernel. As you don't need anything non-free, the vanilla kernel from www.kernel.org is fine. You will have to support every hardware needed to boot (CPU, network card, …) as well as what is needed to access to the NFS-shared root filesystem in read-only. Some more specific needs are Devfs and Ramfs.
You can find a sample kernel configuration here: FoSCuP kernel config
Later on, it would be exciting to try using CodaFS or InterMezzo instead of NFS.