My opinion time:
First off, I'm sure it was hard to give up such a nice car that you only have had for a while, props for doing that to help out your dad!
About the Dmax, I don't know what all the hype is about everyone with a truck switching to a diesel. Sure, I love diesel trucks, and I will have one some day, but as for now, my OBS is my Daily Driver, and it flat out sucks buying gas for it, and having to fix anything that breaks on a 15 year old truck. That truck has been worked, too high mileage for that price, definitely. Highway haulers aren't the best for a daily driver, everything has been through rough weather, you don't know what kind of problems it is going to have.
Diesel is really expensive EVERYWHERE, I have a buddy with a 2004 Cummins, he is getting around 25 MPG, but still extremely high price for diesel, when you need to change oil, you have to buy more oil than for a normal gas engine, and oil is going up too. I like diesel pickups, I will own one someday, I could trade my OBS in for one, but I'm not going to because it would be stupid for me.
You don't like cars....why? Any GearHead basically likes anything with a motor, I never was too big on 4 doors, but I have changed and like some of them now. A gas saving car would be the way I would go for a daily driver, filling up once every two or three weeks and getting 30+ MPG would be great, more cash in your pocket to put towards your OBS, like buying parts, or say...fixing that dent in your bed. You have to go about these things thinking of what the outcome would be, drive a gas saver for a couple of years, doesn't mean lock the OBS up and not drive it, drive it one day a week or so, then drive your car the rest of the time. In the meantime, you are making money for YOU, after you have plenty of money, within a couple years maybe prices will settle, or get better, then trade in and get a diesel truck, maybe by that time even a good deal on a GMT900 or something.
That's just what I would do, we are the same age, but that's just what I would think, there are plenty of nice cars out there man, it don't have to be either Muscle Car or Big Pickup. With the money on that GMC you can buy a brand new Chevy Cruze, it's what I would do.