I put a ebikeling kit on a Trek 830 in 2015. It was 500W 36V rear geared hub for $200. Added a $280 36V dolphin battery and it's been a reliable ebike. Always there if I want it. Just a simple three speed PAS with throttle. 20 mph. No display. No frills. The natural upgrade would have been to upgrade to a 5 level PAS controller with LCD, but I never did,
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I have purchased several more ebikeling geared hub motor kits. I did not like the way their newer LCD kits work with the geared motors. No speed display when coasting. Crude PAS response. I junked their electronics and put on KT controllers. His business has grown, and he is now a big seller, especially for the higher power direct drive kits.
If you want a direct drive motor, ebikeling, voilamart and a whole bunch of no names will fix you up, and you can fly at 30+ mph on 48V. You have to decide whether you want a direct drive or geared motor.
Or you could consider a mid drive. The Bafang BBS02 is about 450? I paid $550 in 2016. The standard kits fit bottom bracket widths of 68-72mm. There are higher cost kits for wider bikes. Better for hills, but you still have to be a bicyclist at heart because you still need to use the gears. Personally, I think the mid drive is easier to install. The cabling is easier and there is no controller to mount on the frame. A hub motor is tricky when you have rim brakes. You may have to dish the wheel to center it between the calipers.
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Grin is very pricey. You need to be making a super bike if you use their kits as you'll be close to a thousand bucks. Even a cheap kit/battery will cost around $500-600. For that kind of money, you could almost buy an imported ebike.