So it’s not that complicated, but you do need to be confident and make your hands dirty. I put extra care in planning my build and cabling it because I wanted a nice looking end result, but there are also others who don’t care as much and have cables everywhere. Neater takes more time, but it’s up to you.
I bought a mountain/hybrid bicycle new (because a used bicycle here costs the same as a new one in Germany.) I went for a name brand but others have had success with pretty much any bicycle, including department store kind of bicycle. Giant bikes are also very popular because they’re easy to find and reasonably priced. Things to look out for are: disc brakes (a must in my opinion because your bike will be much faster and 8KG heavier with motor + battery), and a square tapered bottom bracket (motor only fits these.) Some also prefer a front suspension, but I personally didn’t go with it because it’s extra maintenance and a decent one adds cost. Some go for a cyclocross bicycle with full suspension, but that makes it very tricky to fit a battery and the motor, and will usually require serious hacking and welding/custom part fabbing.
I then bought a TSDZ2 mid-drive direct from China for singles day (11.11, a big sales day in China)
And the battery I got from em3ev which is the best money can buy with a Black Friday and wire transfer discount.
I chose the TSDZ2 because it’s the only aftermarket mid-drive that has torque sensing (meaning that it senses how hard you push down on the pedals and multiplies this effort, when the other motors just sense that the pedals are rotating and applies full force right away), and because there is an amazing open source firmware you can flash on instead of the stock firmware.
As for the battery, I didn’t want to worry about range when going on summer adventures so I got a 17Ah 52V battery (825Wh.)
Buying the tools to put it together cost me about 100.- or 150.- because I was able to plan ahead and buy most tools from amazon.de or aliexpress. Some of the tools I had to buy here because I needed them in a hurry, and that cost more than all the other tools combined. But basically you need a crank arm extractor, a bottom bracket nut, allen keys, a tool to shorten your chain (not always required, my chain for instance was still the right length after the conversion), anti-seize paste. I also needed to re-route my rear derailleur cable so I had to buy a Shimano kit for that (housing + cable) and pliers to cut it to size.
The motor + a 850C color display cost me 360.- shipped from HK and arrived within a week by DHL
The battery cost around 550.- including shipping and a quality 90/100% charger (excluding UPS’s insane brokerage fee of 30.- to collect 5.20 worth of tax due)
On my bicycle, the tricky part was that the cables are routedunder the bottom bracket, but the cable guide holding them in place was too thick to fit between the frame and the motor. So I had to take this out and put full housing on the cable so it would fit without the guide anymore.
On my other bike, the inside of the frame in the bottom bracket area was not machined well and had a bump that made the motor not fit through. Luckily, that frame is aluminum so it’s relatively soft to file down and correct that.
Oh and you’ll need to do some soldering to build the display flashing cable if you don’t want to spend an extra 50.- buying an official one, or to to add a temperature sensor onto the motor to avoid overheating and demagnetizing the motor. I didn’t include the cost of the soldering iron and accessories for that because I already owned it all. It pays for itself very quickly after you’ve repaired a couple of things that you would otherwise need to throw out and rebuy because a 3$ component burnt out.
I haven’t flashed the open source firmware yet because I’m still in the process of building my flashing cables, but I’ll get around to it this weekend probably. Even without the open source firmware, it’s a very fine setup. There is a 7-8% grade to go back home, I used to climb this at 6-7 km/h and arriving all sweaty on top (even in the winter.) With the motor on minimum assist level, I climb it at 12-13 km/h and while it ups my heart rate, I’m not sweaty when I arrive. I could crank the assist level all the way to the maximum, then I barely have to push on the pedals to climb. The bottom line is that with a motor, I can either relax and let the motor do the work when I’m lazy, or I can pedal as hard as I did without a motor but get further and faster before I’m tired.
This is the battery I bought: https://em3ev.com/shop/em3ev-52v-14s5p-jumbo-shark-ebike-battery/
This is the motor I bought: https://eunorau-ebike.com/products/48v500w-tongsheng-tsdz2-mid-drive-motor-kit-with-torque-sensor?_pos=4&_sid=ec2abcd31&_ss=r
This is the amazing open source firmware: https://github.com/OpenSource-EBike-firmware/TSDZ2_wiki/wiki
Good resources to learn more and plan your build are https://old.reddit.com/r/ebikes and https://endless-sphere.com/forums/viewtopic.php?f=30&t=93818. You can see on reddit where people post their bicycles that it runs the whole gamut between neatly tucked cables and rat nest.
And for general bicycle repair/maintenance/removing the bottom bracket/etc, park tool makes amazing videos on youtube.
Happy to answer questions as well.