What are other similar trucks going for locally?
Admitedly, my rigs aren't the prettiest, but I've picked up two OBS turbo idi extended cab 4x4 trucks here for <$2k.
I'd see a pristine one as being worth 3k, 3500 maybe if it's really nice. Remember, it's still 25 years old and you will have to put work into it - seals, injectors and such - within 20K. Just because old rubber is still old rubber.
Not that that's a deterrent to me, but 6k is a lot of money to me.