You don't mention the model year, but later GM van-based chassis have been known to have wiring gremlins that will spike trouble codes. The Pxxxx code series, however, indicates a 1996-year-model or later.
P0380 indicates Glow Plug Control, a fairly common problem on 6.2 and 6.5L diesels.
P0758 indicates transmission shift control solenoid. This could indicate the transmission has been worked hard and overheated, or even worn out. None of which are good. And then again, it could just a bad solenoid or loose solenoid wiring. In any case, the pan has to come off, which means draining the fluid. And GM is not known for transmission longevity in this year model range, either. 240k is pushing it for a factory GM transmission, even for highway miles.
I'm not sure of what you typed for the third code - there is no P011, but there is a P0118, which indicates trouble with the engine coolant temperature sensor or its wiring. In the future, remember that unintended smiley codes can occur by using parentheses too close to what you typed, making it hard to decipher what was typed.
My advice - research all possible causes of your trouble codes and if they can trace back to the Body Control Module, Powertrain Control Module, Airbag Control Module, or anything indicating power loss to any circuit, I would walk away. Ditto if major engine or transmission repair is needed -- possible with the solenoid code.
It could be a simple fuse... but if not, any of those things I mentioned could top $1000 to diagnose, and even more to fix. I certainly wouldn't pay $2500 for it with the transmission being a likely question mark. $1000, maybe. Just my $0.02.