I get the feeling the client would be expecting those tones in the highlights--personally, I wouldn't take on a project like that if I had no halftone experience. That being said--assuming they're on a white shirt (like the ground in the art) I'd think with six heads, a red, orange, deep blue/purple, light blue, black, and grey would be the colors I'd want to go with. If you have great high mesh screens and a decent exposure setup--and if you're good with your non-micro press--I'd take on the printing, but I were you I'd definitely get it sepped and films run by a pro. It wouldn't look as nice on all those smooth gradients, but an index sep would be more forgiving on press. I'd probably do 230 (or 90T, if you're metric) mesh.