Usually the heater units are wider than the standard "square" housing vent fans. You'll probably have to do a little drywall cutting. If you have access to the location from above - meaning if it's a second floor bathroom with attic access above - it will be a lot easier to retrofit the unit.
Make sure that the circuit can handle the wattage of the heater. Otherwise the wiring, should be pretty straight forward if you buy a "single-switch" heater unit. If you need two or more switches (e.g. heat/no heat option, light/heat combos, etc.), things could get a little tricky if you're not comfortable with adding wires.