Diesel Questions

[Stingray42]

I'm kinda thinkin about gettin a 94+ Chev/GMC Diesel.. and have a few questions! My main reason for it is better milage... The Blazer is killing me. The idea comes from my Dads GMC Topkick, as it gets incredible milage, but I just need a pickup! (Reg cab or ext cab shortbox) Preferably a halfton, 3/4 is ok.
What I need to know, is how much better milage do they get than gas? (I have a 350 that gets 14 MPG. I drive about 35 mins each way to school (I'm told they dont like short trips, is this long enough?) My Girlfriend lives 35 mins away. Work will likely involve being able to leave it running. I also can let it warm up in the morning no problem. (and can plug it in if needed too)

Hows starting cold in the middle of the day after it has been running?

Are there any years to avoid?

Just how much do they shake and rattle around at idle? My pops Topkick is pretty bad lol. I'm gonna jump in my buddies for a ride sometime soon.

Is there anything I need to watch for on a diesel? Is it really suited to my driving? I wont tow much of anything, I do a little bit of wheeling now and then, would probably put some bigger tires on it,

Thanks!!