I'd just set up a mixte as a touring bike. Mixtes are plenty strong if a tiny bit heavier than the equivalent double-triangle frame. Not so much the swoopy mixtes and the normal step-through frames; make sure the frame is a true mixte with the top tube/tubes running straight all the way from the head tube to the rear axle.
Setting up as a touring bike mostly requires strong wheels, appropriate gearing, rack mounts, good brakes and comfortable handlebar and seat. Soma has the Buena Vista for about $400 for the frameset. Origin 8 has a mixte frameset for half that. I tend to prefer mid-80's Japanese mixtes as they can be acquired for a hundred bux or so, and some of the components are good enough to re-use. Stay away from the French marques and early low-end Raleighs- they often have incompatible threading and diameters that make for a challenging upgrade to modern componentry.
I'm just about to put a nice Trek 780 touring bike with step-through 18" frame up for sale. It's not quite a mixte, although a similar frame design just without twin lateral tubes. I got it for my wife and built it up as a touring bike with nice components. What's a 780, you ask? It's the European version of the 720 hybrid bike and has chrome-moly tubing. It has rear rack braze-ons, lowrider braze-ons on the fork, and other nice touches. Almost new touring wheels with Shimano LX hubs, etc. I can't recall if it's a 8 or 9 speed at the moment, but if it's an 8 speed it could easily be converted into a 9 speed rear (the freehub will take 8-9-10). PM me for more details if you are interested. I'm in the Philadelphia area, but am willing to ship. Probably looking in the $475 price range. The wheels cost me $200 alone and they are almost new.
'12 Rodriguez UTB Custom, '83 Miyata 610, '83 Nishiki Century Mixte (Work of Art), '06 Specialized Epic Marathon MTB
Is there a particular reason why you need a mixte/step-through frame?
I have used a "true" mixte for commuting with racktop bag/panniers. The main issue is that they become way too flexy under load... really scary feeling when your bike starts to fishtail. Mine is a hi-ten steel Nishiki frame from the early 80s. I have heard the similar comments from other C&V mixte owners. Maybe the newer mixtes are not so flexy, but based on what I've read, I would err on the side of caution and not recommend them for touring purposes.
Now, the advantage one gets with a mixte/step-through frame is for mounting/dismounting. Mixte frames are stronger than traditional step-through frames but definitely not as strong as traditional diamond frames. For loaded touring purposes, a diamond frame is the way to go. This is the main reason you don't see bike manufacturers making mixte touring frames, not even for women. I believe there might a higher liability issue advertising them for that purpose.
Many newer touring bikes have a sloping top tube which does help a little when you have to tilt the bike down if one has hip issues, for example. This is not nearly as ideal as a mixte/step-through frame for mounting/dismounting, but it's better than losing control of the bike due to a frame that's not stiff enough under load.
Last edited by Chris Pringle; 10-23-12 at 07:41 PM.