Al, If you have not already, you should read "Short Way Round". It has a lot of good imformation on not only a round the world trip, but also on bike prep. He started out on a BMW, has switched to a DR650. He is well into his trip and seems to have a pretty good handle on what he is doing.
Your Dad gave you some good advise if you are the type of person to ride a motorcycle around the world. Not everyone is, but he probably knows you as well as anyone. As Mark Twain said it is better to regret the things you did than the things you didn't do.
I feel that for someone on a limited budget, the DR650 is the only one to consider. If you don't change to a 520 chain, include tools and spares, but with all 3 bikes being the same that should not be too big a deal, and is something you would want to do no matter what chain. A lot of people have gone with the 17 inch rear tire, make take a little more planning, and again this is where the same bikes helps.
Enjoy you trip.