Recently I started working as a bike courier and to make my life easier I want to convert my bike into electric one.
Since you will ride quite some distance with your ebike and mid-drives put a lot more strain on the chain and sprockets (I've read of people who had to change their chains every 1,000km or so), I'd advise against a mid-drive.
Geared hub motors are fine, but choose the bigger versions (250W version of the Bafang CST, eZee or Puma/Mac for instance), because their planetary gears and clutch are much more sturdy than in the small geared hub motors.
Direct drive hub motors are quite heavy, but since their design is much simpler, they will run much longer between repairs. If you don't 'cook' them, the only thing that can wear or break are two ball bearings (10 bucks). DD-drives are cheaper than mid-drives or geared hub motors, too.
Whatever hub motor you choose, get a version with cassette adapter! The hub motor versions with screw-on freewheels can be a real pain to mount properly. Also, get quality torque arms! https://www.ebikes.ca/product-info/torque-arms.html
I commuted between 30km and 65km a day for several years now (>60,000km) and went through several geared hub motors. Usually I had a second e-assisted trike, or at least a spare geared hub motor ready to swap, if the one in the commuter trike would break down.
Now, I only use direct drive hub motors for commuting.
If you work as a bike courier, get a direct drive hub with cassette adapter and a battery that has at least twice the capacity you actually need*.
To determine the needed capacity, play with the Ebike Motor Simulator: https://www.ebikes.ca/tools/simulator.html
Don't skimp on the battery! It the most expensive and most important part of any (well made) electric assist system.
Also, get a good battery charger (Em3ev*** got cheap ones with 90% charge setting, they also build high quality batteries!) and don't forget to have a spare charger at hand. If your charger dies, you are stranded.
Probably not in your budget yet, but hands down the best available ebike charger is the (360W, waterproof, programmable) Cycle Satiator: https://www.ebikes.ca/product-info/cycle-satiator.html
I got one of the first available units and use it for nearly 5 years now.
It ain't cheap, but when I count the costs of various 'cheap' Chinese charger's that died on me over a similar time in the past, I've theoretically saved €50 already.
Also, while usual ebike chargers sport a noisy fan, the fan-less Satiator is completely quiet.
*the battery will loose capacity with age and usage and will have a much longer service life (2-3x!) if you regularly
charge only to 90% and only discharge it down to 30-40%. Thats exactly what car manufacturers do with electric car batteries, to extend their service life.
**thats the first tool I use, if I set-up a new assist system or tinker with one of mine (seven to date and counting).
***Em3ev is an Englishman based in Hong Kong, who runs a small family business and worked hard for his sterling reputation in the ebike community. He's actually the only 'Chinese' ebike parts vendor that I can recommend: https://em3ev.com/store
Here is a video of their battery production: https://etrike.wordpress.com/2017/06/28 ... ctory-tour
(tough-out the annoying commentator/vlogger)