Most people seem to quite like the Elektron MachineDrum, and it's what I'd go for if I had the money spare. If you're after a more budget solution, I saw someone do a live PA with the two older Korg Electribes (EA-1 and ER-1) the other week, and it sounded pretty good from where I was standing. I've got the EA-1 - the synth module, and it's pretty good as VA synths go, so I'd imagine the ER-1 is ok as a drum machine. The ER-1 also gives you a synth part, so would let you have some bass noises as well.

From what I understand, a polyrhythm is any pattern or sequence which has several different rhythms going on at once. i.e. a pattern which might fit into 4/4, 3/4 and 6/8 all at the same time. Have a listen to Squarepusher's more recent stuff (e.g. "Do You Know Squarepusher?") for examples.