IF the SSR can accept 5v, you can control it directly.
also, your SSR should have 4 pins.
it should be power > heater > SSR > gnd
on the other side it should be signal and gnd.
the signal and ground of the control side of the SSR need to be in common to the Arduino, but the power side does not.
do you have a data sheet on the SSR ?
Also, you might be able to just use the MOSFET directly for the fan and the heater.
just past half way down is the SSR
the signal side is isolated from the power side.
make a better sketch of hte