Help me out- should I buy this truck and what should I do to it?

[toupee]

Hi all,
Here's the situation. I'm looking to get a truck for my business that is capable of hauling 4 grown men comfortably for hours on end. I know I want a diesel so 1/2 tons are out. I have always been a dodge guy (sorry fellas ) However, I have found a truck for sale locally that has caught my attention. It's a 99 F350 Crew Cab DRW 4x4 auto with 152,000 miles. They are asking $13,500 for it, but I imagine I can get it for $12,750. Seems reasonable to me. My fleet has 3 powerstrokes in it now (99, 00, 01), so I have a little knowledge about them, but not much. I know the truck needs some steering and suspension components since they are worn. I also noticed a good bit of oil around the side of the block by the oil filter. Oil on the tranny pan sides too. The differential has a leak where the yoke connects to the driveshaft (hope that makes sense). Other than that it has only minor cosmetic issues and worn leather seats. Haven't heard it yet, but was assured it runs fine and the drivetrain has no known issues. I plan on having a pressure test done at the dealer if I'm really going to get serious about this. Anything else I might need to do?
Assuming I get it, I want some input as to what I should look into doing to it. I'd like to get the best mpg I can out of it. I like to add propane to my trucks. I work at a propane dealer so it's really a great cheap way to up mpg. Do I need to look at replacing the turbo with a bigger one to keep egt's down and allow for more propane injection? Should I be thinking about new tranny options? We regularly have trucks get stuck in the winter and having a truck built tough enough to pull out a loaded down 16,000# F550 would be nice.
Unless we get to ridiculous proportions, money shouldn't really be an issue here since I've also looked at $30,000+ newer trucks.

[Evil Eye]

Check to see where the oil is coming from, but doesn't sound unreasonable. The differential leak doesn't sound good. I'd make dam sure that the dealer takes care of these problems before driving it off the lot.