Your budget is enough for a U1300 (newer) or 416 (older). Your life will be way easier if your Mog is over 25 years old. You'll also want to check with your particular state to see if they'll be any impediment to registering it; a couple states make it difficult to impractical.
A 416 is a big diesel truck, a U1300 is a giant diesel truck. They are comfortable enough, except for the formidable noise, but they will generally struggle to cruise at anything past 55 mph, will be loaded-semi-slow up steep grades and most are not a good choice for routine distance travel in the U.S. If you do get one, though, spending the money for a turbocharged 1300 can be worthwhile if you expect to travel in elevations above a few thousand feet.
There are, in the 4xx series, several variants besides the 416, but be wary of anything with a shorter wheelbase. They look great, but can be downright dangerous at highway speeds (which most of them can't go anyway).
Most near-antique used trucks will likely have reliability issues and Mogs have their share, including components, like the portal axles, which can be prone to failure. Nor are they cheap to keep running, and it's near impossible in most areas to find Mog-specialist mechanics, often making it a DIY job to keep it running. I suggest that potential buyers figure their Mog will end up, at best, 50% transportation device, 50% hobby . . . which may be fine with you, but don't expect to interact with it like a normal truck, not even a normal 25+-year-old truck. For good or bad, Mog journeys are adventures; good if you've got four months to drive to Costa Rica, bad if you have to slog back from the desert in Sunday afternoon freeway traffic.
One thing to think about re: your motorcycle hauling is that the bed of a diesel Mog is usually quite a long ways off the ground, four feet or more in some cases. That's the price you pay for portal axles and huge tires. Could be a real pain getting the bikes in the bed.