[captain harlock]

Exactly, picking the right motor comes with experience.
If I were a fresh hobbyest in MT brushless setups, I would've gone for even a 1950/4 just because its insanely fast. However, soon you'll realize that you are running both the motor and the controller at their hot steak.
The LST2 is impressively heavy and you need to keep the motor and controller at their best effeciency.
I dont think that its impossible to use the 1950/7 with the LST, though you can use it and get some serious speed with the right gearing and number of cells, but your truck will run with more power if you desire more speed and torque from your motor and controller without making them beg for mercy if you choose a motor with less rpm/v.
The 1950/9 is a centerline between speed and torque. You can run it in the LST2 with 5s for nice speed and torque with moderate temps or you can go for 6s for some 45-50mph speed runs. You can easily use even a 7s pack, but you can no longer handle the truck with ease like you would with a lower cell counts.