I would recomend to use a PIC with PWM output, that way you
only has to set freq. and dutycycle in sw. (Very simple)
I would try out the freq for the application, but a good place to start is 0.5-5Hz in your application, since you need to go lower the main freq.
The wave form is of couse a simple square wave to control the relay, and the duty cycle is simply set to 0-100% dependig on the wanted output power.
If you need a ADC aswell I think 16F870 is a nice choice even if it's a huge overshot, but the advantage is ADC, PWM and the possibility to use a ICD for developing. You even have a serial port if you want to have a PC i/f
/Froggy