QS138 is the popular choice that most people have but they are mostly in single seater karts. I think the motor 'can' handle a two seater, depending on how you drive. From memory some people have pushed the motor up to 25kw output. But if you're driving it hard like that it will likely overheat. A better choice might be the QS180, it can handle maybe slightly more power but is a larger motor so can dissipate the heat better.
For the controller, I think fardriver is more common though there are a few other popular brands. NOTE which ever one you get make sure it has compatible wiring with your motor. If you order a motor and controller bundle you shouldn't need to worry about this. You can order direct from QS Motors or aliexpress (I've done both). Since you're pushing a lot of weight be sure to get a controller that can handle lots of amps, maybe 1000 or more.
Just when you think have spent a fortune already, you'll need to buy a suitable battery. This will likely cost more than the rest of the kart. Don't scrimp and cheap out on the battery. Having a high power motor and controller can't be high power with a crappy battery. 72v minimum or higher (make sure the controller is rated for higher voltage as well). Armorge is a popular brand. Batteries will be listed with rated current and peak current. You want something that easily peaks out above your controller, why? because the peak current listed is when the battery is fully charged. When the battery starts to drain (below half) they usually have lower power output.
So a quick summary
QS138 for fun with the kids
QS180 to drive it like you stole it
Get the best of the best battery
Your bank account will hate you
You won't care about the bank because you're smiling
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