Thanks everyone for the reply. It surprises me how dominant the Bike Friday's are among bike tourists when I expect they are a tiny fraction of the folding bike market. But they make a quality product and pay attention to the details that matter.
One of details that I am struggling with is how to use my panniers on a folding bike. Bike Friday makes this easy because they elevate the rear rack far above the rear 20" wheel so maybe its height off the ground is not much different than the rack height with a 700C wheel. I don't think this is true of any other folding bikes and if it is I don't know if the rear rack on other brands could really support the panniers. It seems that pictures I have seen of people touring with Brompton's show them with a special large single bag either in the front or back. A large single bag could be an advantage on airlines as it would be easy to check. 2 panniers are 2 bags and that can be expensive to check on an airline. I was in that position in Paris, I wanted to check my bags and go explore but I didn't want to pay the ridiculous baggage charge. But if you plan in advance it probably isn't too difficult to figure out how to strap them together and put the inside a nylon duffel to make them 1 checkable bag.
The other detail important to me is the rear hub. I prefer not to have a rear cluster and derailleur. Since I plan to mostly be in urban environments I would like to keep the grease off my hands and I've seen a lot of rear derailleurs damaged when bikes are transported and a geared hub seems like an easy way to avoid these problems.
I've been leaning towards a Dahon which I can get on the second hand market for about $600 with an internally geared rear hub. There are plenty of Bike Friday's for sale but the tall frames are extremely rare. I need a folding bike equivalent to 34" standover height. Bike Friday makes 3 different size bikes and that is probably why they ride better than everyone else's universally sized folding bike. But it makes it hard to find a Bike Friday on the second hand market if you are tall. I prefer not to spend a lot of money since I am getting on in years and it is hard to predict how many more bike adventures are left.
This thread has been useful and maybe I will think about how to make an elevator for the Dahon rack or move the mounts on my panniers. I will also look harder for a tall second hand Bike Friday but finding one with an internal hub is unlikely.
My trip itinerary is still fuzzy. Using what I learned last year I am thinking of flying to Spain a week and a half before the first of the month and then shop for apartments and a Spanish language school in either Seville or Granada. Then rent an apartment for a month and explore Spain on the high speed trains on the weekend. The bike would be used every week day to commute to school and on weekends it would get me and my luggage to and from the train station and hotels and around whatever city or countryside I found myself in.
Thanks again everyone!