You will need to decide 2 stroke vs 4 stroke and then determine what type of drive system you want to have.
Friction drive is a reliable basic system but doesn't always perform well when wet. Sprocket driven single speed design is probably your next option.
The GEBE system seems very reliable and well liked by those who own them.
Next I'd say the multi speed systems using a jack shaft which can provide you a broader useful power band.
And then you have a few systems which can allow you to power through your pedal axle and have your existing full gear range available to your engine.
Sometimes it's functionality that drives your decision and other times it's cost versus functionality that guides your choice. There is no right choice, there is just your choice.
Like houses, chances are you'll end up owning more than 1 across the years.
As for where to get it installed that will depend on where you live. You might consider buying a completed bike with motor from one of our member cyclists who makes a business of creating these fun tools/toys/transportation devices.