So the motors I was looking at have a max torque of 619 oz-in (or 17.5 kg) @12VDC. And that servo is 40 kg torque.

619 oz-inf is 4.35 Nm, 17.5 kg-cm is 1.7 Nm.

You need to sort out which, and I'd strongly advise using SI units throughout as its much

easier and you won't make silly mistakes.

First estimate total robot weight, then for a 45 degree slope that means your wheels have to push

with 71% of that force, so that gives a handle on the torque needed (allow a factor of 2 for

friction and drive-train losses though).

If you know how fast you want it to move up that slope that will directly give you the total

power needed (again, factor of 2), and thus you can size your motors. Do not choose the

motor first and hope, that's just silly. Find out the power and torque requirements (even

roughly) and then choose motor(s). Only then choose motor drivers and battery system.