High mileage may not be a problem if it's well maintained and hasn't been driven into the ground.
Not sure where you live, but quite a few trucks come out of Alberta from here and those have been driven in some pretty sketchy conditions... But even still if they were well maintained then their generally fine.
I'd totally do a carfax and get it brought to a local shop to give it a once over.
Also, trucks (All trucks, not just these... As I've found out) are quite expensive to maintain so if you can't do it yourself make sure you've got a couple grand sitting around at all times to handle the inevitable.
GMC Sierra 2500hd 2004.5 now with ARP studs