I'd say they're absolutely good for AROS. I just suggest to avoid anything too old or too much underpowered to give a good experience. Don't forget different processor architectures behave really different at the same core frequency. For instance, a 1.8 GHz Sandy Bridge processor will be better than a 2,4 GHz Pentium 4. But that doesn't matter at all: what I say, in the end, is just avoid to use those old 400 or 500 MHz processors. The fact that AROS can run on them doesn't absolutely mean you wish to use AROS on them.
I think the approach is wrong, perhaps focusing in on a reasonable performance gfx card that AROS supports would be better as most technoids would associate a reasonable system with that level of GPU performance. ie, if the gfx card is PCI-E and not AGP, a 233MHz pentium would not be considered.