I'm not planning on getting a CDL at this point, and a F650 is a noticeably larger footprint than a 550. My budget will be around $10k. I put about 5,000 miles a year on my 350. When I replace my current daily driver, I expect to decrease the mileage on the truck by quite a bit. There are some hills around here that I would drive through, but they are full of 35mh curves. So, while I am actually concerned that a 7.3L PSD is gong to be a little on the week side for the job, I'm more concerned about being able to go down hill SLOWLY than uphill quickly.

For 40-45 miles each way, if it takes me an extra 15 min each way to make the trip its not a big deal.
As mentioned, this really isn't going to be for business use. Its primary use will be transporting logs for firewood, and hauling equipment (tractor and loader) to and from the farm to either load logs, mow pasture and lanes, or do some other kind of work on that property. Occasionally it may get used to haul equipment and volunteers to disaster sites. All told, it will likely get used 8-10 times per year. Anything else is just going to be an opportunity to start it up and let it run so that its not just sitting there. If it weren't for the fact that the F550s I'm finding are in the same price range as a 250 and 350, and are often in slightly better condition because some punk hasn't tried to hot rod it (yet), I wouldn't even consider anything bigger than what I already have. I have no expectation of being able to replace my current salary with it. If it does a couple of jobs a year to pay for part of its insurance and plates, great. If I can step away from this rat race in a few years, it may be useful for generating some supplemental income. My life style doesn't require a vehicle this size, but it does occasionally require something bigger than a 1/2 ton. If I'm spending pretty close to the same money either way, I THINK (but could be wrong) that it makes more sense to spend the money on the bigger vehicle because it allows me to complete my work in fewer trips and with less physical effort on my part. Much easier (and faster) to dump logs from a dump bed than to climb on and off a tractor/loader to hook up and skid them off a trailer 1 or 2 at a time, and much easier and faster to load them with a loader than winching them onto the trailer one at a time so long as I can tow the tractor/loader back home on the same trip.
If I could upgrade the payload of a 450 dump truck to handle 10k and still pull a 10k or 14k trailer, that would be even better.