for the SSR i linked, the sheet says 3 miliamps leakage current, but that also is under "ac to ac relay", which that is not... its dc to ac. the range they specify for this model line is 3 to 20 miliamps, so if you are calculating worst case scenario, i would use 20mA to be safe.
is it normal for a relay to a have a coil voltage that varies between 12 and 220VAC?
there are two parts to the relay: the load and the trigger (also called "out" and "in", respectively). the load is going to be your heater element, so you want something rated at either 120 or 240v and the correct amperage (you can also get relays that switch DC loads).
the trigger is going to be whatever signal you are using to activate the relay. most often PID controllers output a signal of around 8 volts DC,so most common SSRs have <12vDC triggers. you can also get relays that trigger when you apply 120v AC.