That class of computer is actually quite popular with certain groups these days. There's a popular series of machines produced by the original designer, as well as a huge variety of compatible clones, and lots of peripherals available. Actually, you've got a high-end device if you've got 64k to play with; most of them just have 32k. Of course, I've yet to hear of people trying to run something Unix-like on an Arduino, but it's certainly a platform which needs low-resource programs and provides unusual benefits over platforms that aren't so constrained.