Lots of testers use celerons to test with and it seems that their results are just as valid. Tony Thomas test on a celeron and also uses almost exclusively 1'1" time limits and his results are very close to the truth.
There are engines that do not do well at blitz on slow machines, however, since I am only testing for a relative difference between beta versions and not for accuracy of rating there should not be a problem.
And bare minimum nodes as I originally described them should even be less expensive than what has been suggested so far. The cheaper the better. So, a simple celeron (sempro?) box for US $250 would be fantastic!

After having said all that, it would be a shame to not run 64 bit test! But once again everything is relative and if the 32 bit version does better then the 64 bit version should also be better.