I don't recommend going for the cheapest lowest spec thing you can find just because the MorphOS license is tied to the hardware, and is difficult to transfer if you decided you need more capability...
I went through this with efika. Sold the MorphOS license and basically threw in the machine for $1.
Its like putting expensive rims on a clunker 
The G4s have at least the advantage of being well upgradeable. I think a Quicksilver PowerMac G4 can get equipped with 1.42 GHz cpu cards or even faster ones.
From my personal experience I can very much recommend the Mac Mini G4 1.5 GHz (very fast, tiny, not too loud and low wattage). It's a bit harder to obtain than the 1.42 GHz minis, but it's worth it. And if you happen do dislike MorphOS (I doubt) you can sell it off easily again via ebay and get probably about the money back you spent to try it out.
