I'm a mad scientist engineer and applaud your idea
Some unstructured thoughts:
You'll need closed loop temperature control with a sensor ( a thermistor in the airflow ) to allow control of the energy being fed in properly. Look up PID control, maybe browse some "diy 3d printer design" sites and look at how they control the temperature of the hot end of a printer.
I'd go for electrical heating as it's the easiest to control by far.
I forsee an arduino in your future, don't write your own PID code, grab some from an open source 3d printer controller, get code with autotune function.
It's difficult to have fine temperature control of something with a large thermal mass, there will be under and overshoots if you try to make it too quick to respond, so go for a slow cautious warm up of the heater perhaps...
Consider where the exhaust heat goes, no point burning a hole in your sofa because you weren't paying attention to this universe for a few minutes.
Maybe have a "dead man switch" to cut the heat when your hand relaxes. Be sure not to over dramatise the name of this device when telling the person who is about to trip about it... maybe "very relaxed person switch" is better
Maybe power it through an RCD the first few times you power up your frankenvape
That's all I have for now, the day job calls, safe tinkering!
cheers,
Frac4
P.S. if there's a local "repair cafe" for you then it could be a good source of tinkering bits.