for $500-600, I'd look for a good used road bike like the Trek 1500, but from your description I think a cyclocross bike would make you happy. They have similar geometry (not exactly though, so need a good salesman), handlebars etc to road bikes, but with fatter tires (good for commuting, bad pavement or decent trails) and if you ever want to go fast you can put skinny tires on them. Cost is similar to a road bike, so you're still probably better off with a used Trek XO-1 or Kona Jake the Snake. There's also a cyclocross forum on here, so you could post there.