If they can pay for it, let them get what they agree one. If you’re not a financial partner in this, and your daughter is 20, and legally an adult, there’s little you can do to stop her.
The Tracker is pretty sure footed for an SUV, it’s pretty underpowered too, so it’s not like your kids are going to be drag racing it. It’s also reliable, and since it was sold all over the world, servicing and repairing it should be inexpensive. The bad news is that the Tracker does not hold up well in crashes at all. It’s pretty small and lightly built, it’s not safest car around.
As for sharing a car, I’ve never seen it work well between siblings of the opposite sex. There will be plenty of conflict about who gets the car on saturday night.