Fan motor simply keep the old heater resistor, or go hunt in the scrapyard for an E36 BMW heater controller blade, the switch and wiring loom for it. That uses a MOSFET, and it will mostly fit in there. Also is easy to repair if the motor kills it. While you have the dashboard apart take the existing VW blower motor out, take the drum fans off it and strip it and relubricate the bushings and check the brushes for wear. Use a good quality mineral oil, and heat the bushes up with hot air before dunking in the oil to allow it to soak in, and put a few drops on the felting reservoirs inside the motor.Worn bushes and brushes kill the controller pretty fast, and that resistor pack is cheap, though I have modified my one to use the much cheaper post 2002 VW one, as it is just reversed, and you just put it in the old housing. $3 for the pattern part as opposed to $30 for the genuine one. they tend to rust in humid climates.
As to the heater, I really would recommend using the full EV battery voltage, whatever it is, as that will mean you do not have to use 50kg of copper bus bar to power the heater, and the control is also easier as you can use IGBT switches ( a 3 phase module with all 3 phase switches in parallel is both cheap and easy to mount to a heatsink) to switch it, and can use a low frequency to switch it as well. plus you can get a 2kW heater element made, or find one online, that will be able to fit in place of the old core, and the wiring will be thinner and easier to route. 2kW at 12V means the car battery that drives it needs to supply 200A, and that is equivalent to the starter draw, so you will need an inverter from the HV rail to supply that as well or the battery will last about a half minute before buckling all plates.
2kW at 100V is a pretty standard finned air heater, you should be able to find one that fits, though please be sure to select one that has a surface temperature of under 100C when in operation, and add a thermal cutout on the inlet side of 115C as safety for failed fan, and a 130Csingle shot thermal fuse as well. You probably could fit a standard PTC element one there, giving thermal control as well, but probably you will not get 2kw in a PTC element.