I'm not sure the 'last for days at slow speeds' requirement is realistic unless you've got a way to recharge during the day. A really expensive, really high capacity battery on flat ground at minimum assist on a very small hub motor might get you 100 miles of range, or a little more.
I've just finished converting a mountain bike with a BBSHD mid-drive (really only slightly more involved than a hub motor install if you're using a PAS sensor on the hub motor), and I'm in the middle of converting a kickbike for my son using a 500w Bafang hub motor.
Both came as a kit with e-brake cutoffs, display, throttle, motor controller, etc. I had the mid-drive installed in an afternoon (getting the bottom bracket clear was the bottleneck there, and I ended up having my LBS help out when the tool I'd bought proved inadequate). The hub motor install took about an hour and a half due to some clearance issues, but I have yet to decide on locations for the motor controller (the mid-drive motor controller is built-in) and battery. I'm not a mechanic of any kind, but fairly practical and have some basic tools around.
I chose the mid-drive for my bike because I wanted to be able to take advantage of a 3-speed internal geared hub, which will give me the equivalent of a 16, 24, and 32t cog at the rear. I'm currently running single-speed with the stock 46t chainring and an 18t rear cog, which makes pulling away from a light a little tough, but tops out around 33 mph. I may drop down to an aluminum 42 or 40t chainring, which should more or less maintain my top speed in 3rd gear, while giving me more low gear options for torque.
For the kickbike, which has no drivetrain, the hub motor was the only game in town. I went with a geared hub motor in order to still be able to mount a rear disc brake, and because the rear wheel is a 20" and might not fit a larger direct-drive motor. It's got its limitations, as it's unaffected by gearing, but should work well in that application.
I'm only familiar with Bafang gear, for the most part, but lots of others here will have great suggestions. Good luck!