1) Vehicles are expensive, but are much faster, especially on road and in good terrain. You might have picked a sport car to climb mountain.

2) Report to Vehicles Framework. This mod does not make the UI.

3) There are most likely objects on the way.

1. Yes vehicles are expensive, but they don't cost anything besides time to maintain and repair afterwards whereas animals have to be fed, and cared. You can carry Pawns in vehicles and the combat ones can provide some nice support during a quest or general home defence.

2. I haven't really had a problem with it so no comment.

3. I had the same issue. No idea if a save and reload fixed it, or whether it just bugs out if you build it inside a garage. After deleting mine and getting a new one outside the garage, it happily drove inside no problem.