Advice On This Truck Please...

[BigBlocksRule]

Mileage affects more than the drivetrain...suspension bushings, etc.
Look for sealant between the intake and top front of the block that would indicate the intake has been off. If so, then at least the junk factory intake gaskets would be gone.
I have a '97 crewcab with a 350/4L80E. This engine has as good of oil pressure as any new one I've ever owned.
It's near Christmas, stick to your guns. If he still has it in January, he'll probably get his asking price once folks start getting their tax return checks.