I would look for something that seems over engineered, most frames could but shouldn’t handle the stresses produced by the motor, an mtb frame sounds about right
I would like steel or titanium because steel is more predictable than aluminum and titanium is more durable, i would probably steer clear from carbon because they are engineered for certain stresses and the tubes get subjected to new stresses.
If i choose steel i wouldn’t look for a high end steel frame because they are a little less predictable because they are hardened more
(but they might prove to be a better choice)
Choose a frame that hasn’t been used much so you know it has some life left, metal fatigue is a thing