2001 EZGO / PDS burned up motor and controller

[BCR840]

I have a 2001 PDS/TXT / 36V cart with a lift, back seats, 22/11/10 tires, 400AMP Alltrax controller. The kids were riding around the other day and noticed a clicking sound from under the cart. They were headed down a hill and said it felt like the motor was catching and trying to stop. They pulled over and smelled the electrical burning and found that the motor was to hot to touch and was also hot near the controller. At this point they called me (Dad) to drive up and check out. I could not get the cart to move after switching to tow mode. The back wheels were locked. Finally got it on the trailer and back home. Opened up the controller cover and found the Alltrax burned beyond repair. The wires were all okay. I disconnected the wiring and then removed the motor to free up the wheels, to be able to get it in the garage. All Trojan battery posts were nice and tight, no melting on the posts.

Now for my questions.....

What would cause this?

What motor/controller combo should I go back with?
(I live in GA and have decent hills, with at 3 kids riding)

What should I check before installing new parts to keep it from burning them to?

What kind of money are we talking about?