Re: Certain type of derailer for a shift kit?
The above post says it well.
I might lean towards a newer model with better chain cogs if possible, but any should work if properly adjusted and aligned. Pay special attention to the limit screws since damage happens MUCH faster under motor power.
If you want the best, find one with actual ball bearings in the cogs. (Good luck. I've seen two in 40+ years riding)
If more is better, then too much is just enough.
If you can't afford it, build it yourself.