Quoted: Would prefer shaft drive, air cooling, and overall the least amount of maintenance necessary. Would also like it to get 50mpg. |
1992-2003 Honda Nighthawk 750 is almost a dead match for what you want. It has chain drive, but otherwise, it's air-cooled and the valves are adjusted automatically. 50MPG isn't hard to come by on them, either.
The older Nighthawk 700S actually even has the shaft drive. They're a great bike with a pretty rabid fan base, as well. They don't come cheap if they're in great shape ($2500 or thereabouts) which isn't low for a bike produced from '84 to '86, but it's not a bad way to go.
Just realize that any of these bikes will be a compromise. For your cheap, you'll get a poor tire selection, squishy suspension, and brakes that aren't the greatest in the world. But reliability, ease (or lack) of maintenance, efficiency, and overall ability to withstand a lot of abuse and day-to-day riding are the high points.
How cheap are we talking here? An older K75 BMW might be doable for about $3500... the brakes and suspension are better than the old Japanese bikes, and that little triple will run forever. But you'll have to do your own valve maintenance. Again, 50MPG is practically a given.
Old small-block Moto Guzzi perhaps (V50/V65) if you're feeling eccentric - simple, run forever, easy to maintain, and 60MPG is within reach, too. Brakes and suspension were usually better than most anything else from the period, as well.
There are plenty more... it all depends on what you have to work with, and what's available at the time you happen to have the money!