That's certainly more than enough power for 3mm material, but I am concerned about the substrate itself. All of my experiences with polystyrene materials have resulted in fire. If you can get some test cuts done before you buy, do it. If the manufacturers won't do a test, send me a PM and I'll try on my machines.
The Chinese chillers should be more than enough. Neighbor is running his 130w machine cooled only with a small radiator, and we're in Florida. You should have no problems with an additive to prevent freezing, but you'll want to be very sure it doesn't happen or you'll crack a tube.
Good luck with your new venture!