Currently it is hard to find an inexpensive 3-speed IGH step-through pedal bike to convert, most all of the brands that sell those like Linus, Brooklyn, Public, Detroit, Breezer, Worksman, BikesDirect, all the big box stores, and local bike shops, etc. are sold out through October due to everyone wanting to buy a bike for exercise.
Does it have to be a rear wheel hub motor? The simplest would be to add a Hill Topper Sprinter throttle only front hub motor kit to something you can find on the used bicycle market. or try online Amazon, ebay, craigslist, etc
Or you could try a budget generic Chinese brand ebike like a
Nakto Camel, that bike has no IGH it uses a derailleur, but it's one of the simplest ebikes with no display, just one level of pedal assist, the motor is either on or off, so you use the 6 gears or throttle to control your cadence/speed. Given how easily mail order bikes are damaged in transit I recommend paying a bike shop to assemble it so they can check the brakes, gearing, etc. and replace anything broken in transit or assembled incorrectly.
On a point of information you generally won't find an IGH & rear hub motor combination as they both have to occupy the rear hub, there is only one motor that combines an IGH inside the rear hub motor, the TDCM IGH sold by Grin Tech, but that is outside your budget.