Both the head and the table should be properly controlled with a PID PWM system. I assumed they were but as I say, I know nowt about Repraps. You only need one transistor to PWM the heat no matter what the voltage and the advantage of using either AC or DC 120 or 230 matters not, no need for a regulated supply, it just doesn't matter how crap the supply is if it is PID PWM controlled then it will perform correctly. Better to use a seperate board as you say but a small PCB with a PIC that is just todl what temperature you want and leave it alone is by far the best way or you could just buy a PID controller for a few quid more.