You know, depending on the non-profit, I would be sorely tempted to spend $500 out of my own pocket just to purchase a stable machine that could serve the purpose. Hundreds of GB of diskspace are cheap, and even if that's not big enough (at 1+GB/day one can still fill a "big" 200GB in only 6 months, so you may have a bigger "huge" disk than that), you can still have that brand new, stable, non-crappy disk for temporary writes.

That said, if the two machines are stable enough themselves, NFS seems perfect to me. NFS is generally stable enough for most purposes.