+1 on a 26" LHT. If the 600 miles are on rugged terrain (do you know?), you might be more comfortable on a Troll (also 26" wheeled) as it's a mountain bike.

BTW, I hardly put the above $500 touring framesets in the "last one forever category." If that's what you're after, you will find yourself quickly wanting a custom frameset for some obvious reasons: perfect fitting, desired frame geometry for on/off road, S&S couplers, color options, braze-ons for different kinds of brakes and other features, etc. If you're thinking of doing lots of international touring, personally I think two sensible requirements are 26" wheels and, believe it or not, S&S couplers. The latter will pay off if in just 2-3 international trips ($400 per R/T) and will allow you to fly your bike on puddle jumpers, take trains and buses.

Surly offers the 26" LHT frameset with S&S couplers for $995. Many wish they offered a disc brake option. It's the best deal out there for someone who is serious about international touring in the coming years. Just something to think about!