If your budget is $600, then I would start searching for used bikes/frames on eBay, Craigslist and other on-line sources. Make sure you know what size you need. If you want eyelets for racks and fenders, I would search eBay for touring bikes, as well as various Japanese and American brands that made touring/sport touring frames back in the 1980s. Some good brands to follow include Miyata, Nishiki, Panasonic, Fugi, Shogun, Centurion, as well as some of the older Trek and Specialized steel frames.

If you can increase your budget a little, or have some parts that could be fitted to a decent frame, I would look at some new frames such as Surly Long Haul Trucker, Soma Smoothie and ES, Salsa Casseroll, Kona touring model (don't remember the name), Rivendell Bleriot. All of these frames are reasonably priced and have eyelets for racks and fenders.