Another vote on the Ford. The 7.3 powerstroke is about one of the best motors ever used in a truck, my 96 f250 has 320k plus miles and still rolling strong. I'm planning on shimming my injectors this weekend (after a ton of miles the injector seats wear, causing tough cold starting and power loss till warm, not really a problem in fl, but basically kinda rebuilds them), and then doing the turbo pedestal o rings (if it looks like the rear main is leaking, it's those o rings up top). They are very conservatively rated too, so if you are going to risk an overload ticket on a weekend or something, the truck won't mind (not that I would ever do that lol). I'm not familiar with Isuzus, but I would be very impressed if they even came close to what that Ford would do, and for relative cheapness in parts.
If you aren't super pressed for time, I would talk him off that price tho. 6 month flip sounds like he got it super cheap and wants to make a quick buck, especially with the fleet maintained thing (he bought it at an auction). Look it over real good, make a giant deal out of everything you can oil leak wise, and make sure the hoist works perfectly, no leaks or sticking. Throw a fit about everything, and I would bet he comes off that price (bought a truck for a tree business 6 months ago, but doesn't have a tree business now, that sounds fishy as hell to me)
