$1800 is a great price assuming it is in good shape. Mine was around $2000 for a great condition one.

1. Not sure about the pops, but mine activates when the Trex is involved and during some of the modes.
2. Never had an issue with my Trex, I replaced it with the custom one by winteriscoming and its amazing looking now. As for its function, several of us disable the left\right motor as it is prone to having issues. Several people also use the new code that reduces its use to help save it. You can replace it but its pricey. As Chitownpinball said, basic knowledge will be enough to work on the Trex.
3. I don't completely understand this sentence either, but based off a guess I'm assuming you meant that the Trex ate the ball, it disappeared then appeared on the playfield again. If so then that is normal and one of the highlights of the game for me. Trex will eat the ball, then it will shoot out of the right VUK back onto the playfield. This is a time the shaker motor is active.

Hopefully some of this was helpful.