I would avoid Mercedes engines like a plague. The Ford 6.0 has earned a bad rep as well, but they can be improved upon. I personally wouldn't buy a Cat engine, but they're OK if you don't mind visiting a Cat dealer (not always easy to find in small towns) for parts (as mentioned, usually at a higher cost).
The Cummins engines, both the 8.3 and 5.9 are pretty solid engines, I wouldn't expect more than about 300 HP from the 8.3 and about 200 or so from the 5.9.
IH engines, the DT466 (very common) and T444 (also common) and Ford 7.3 (same guts at the T444) are also pretty solid. Their DT408 and DT360 cousins are good (The DT408 is almost the same as the DT466, just a smaller displacement internally).
Cap'nHooligan asked if one could get 600 HP from a Cummins 8.3. The short answer is "Sure, you can", but at what price? A couple days for a tractor pull setup. I've seen Dodge trucks build the 5.9 for insane HP, but they don't last long. For our purposes, you'll want to consider something considerably larger, like an ISX Cummins (Somewhere around 15-16 liter engine) but they can be factory spec'ed for up to about 600 and will last many, many miles at that rating. The Volvo I recently turned in had that engine and was showing close to 800K miles, and the engine still had a lot of life left in it.