The vehicles of Ghost Town questions

[Caralampio]

There are two vehicle acquisitions revolving around the Ghost Town quest. One, a Quadra bandit. If you help Panam kill Nash, the Quadra will become available to buy for a lot of eddies from Dakota. There are lots of other cars you can buy including Quadras so this is not too exciting. If you don't help her kill Nash, Rogue will be pleased and gift you the car (wow)!. The question, does opting not to kill Nash affect the Panam questline, i.e. will you be blocked out of any quests? What about your romance possibilities?
The other vehicle is a cool sports car (don't remember the name) which pops up inside the cave where you kill Nash some time later and is also free (yay!). The question, does not killing Nash affect your chances of getting this other unbeatably priced car?
(Another thing to take into account in this situation is that Nash drops an iconic rifle. But let's talk about the cars).

 

[KuroganeHammer]

Like you said if you don't kill Nash you get the Bandit from Rogue. Not killing Nash does not lock you out of any of the following quests with Panam. This doesn't lock you out of her romance I am pretty sure.

However there is another car in the game that can be obtained exclusively through the Panam quests line: A grey and red version of the Mizutani Shion "Coyote".
You have to refuse to go with Panam and her friends in their attempt to attack the Millitech convoy holding the Basilisk. If you do this, Saul will text you asking if you know where Panam is. If you do tell him what she is up to, Saul will reward you later with that special Shion variant.

This is the only way to obtain it, BUT Panam will be pissed at V which will then permanently lock you out of every other quest with her as well as the

Spoiler

Star ending attacking Arasaka with the Aldelcados.

One unique (and admittedly cool looking) car variant vs every other adventure with Panam, your choice.

 

[Bogmore]

If you don't help do you still get the Caliburn? I've always helped her.

 

[LeKill3rFou]

If you don't help do you still get the Caliburn? I've always helped her.

Yes, it "only" require to complete Ghost Town I think (so whatever your choices, the caliburn is available)