Thanks for everyone's feedback. I really like the idea proposed by 14R to mod my Downtube Mini (thus sticking with a bike I already have and enjoy and saving lots of money). I also have an 8H--would you suggest the same modifications (54T chainring, faster tires, and multi-position handlebar) to it? If so, I may mod the 8H instead--it's got the integrated rear rack, larger wheels, and may be better for touring anyway.
Questions:
1) I have no knowledge of tires--is there a particular brand you'd recommend for the Mini and the 8H?
2) For the multi-position handlebar, are you recommending a butterfly bar or some other type (e.g., drop or moustache)? I found an article about them here:
http://sheldonbrown.com/deakins/handlebars.html (again I'm no expert on this topic).
3) Would installing a multi-position handlebar compromise the fold? My Mini folds with the stem and handlebar between the wheels. On the 8H, the stem and handlebar fold on the outside, so I suspect it would be less of a problem to do the mod on that bike.