a couple things I've picked over the years (mostly by reading posts on Bikeforums.net:
Mixte frames: Generally, over the years, they occupy the lower end of the bike builder's catalog... meaning most frames are heavier steel. If you are looking for a fast bike, they may have that disadvantage. (on the other hand, a well designed mixte is way cool). Personally, I find the update mixtes with flat or flared back riser bars have an aesthetic appeal over drop bar mixte but just my 2 cents.
Soma makes a new mixte frame set for $450 dollars in a number of sizes.
650B is totally cool... but I have a couple and I just dig them. Certainly a wide/tall 650B tire (40MM +) will negate somewhat the drop in standover...But the
Maxy Fasty is a nice tire, slick, 32MM, and high pressure - good for fast road riding.
650B does have
wider tires so measuring up a frame candidate is key. Get your metric measuring tape out and keep it in your pocket along with a print out of the following:
http://650b.webs.com/conversions.htm
I keep this stuff in my pockets and if I happen upon a C-List ad or a bike at a yard sale, I can measure up!
Considerations: Going from a 27" wheel to 650B is a looooong reach for the brakes. 700C tires are better, so a bike with 700C originally is easier on the brake reconfig.
For that matter, Trek has made some really nice WSD (women specific design) racers for some time. My wife (4'10" on a good day) has a Trek 1500 (45CM?) with 650C tires. She loves it. It's not a new bike, so perhaps there are a few used ones out there for easy money.
Scrounging for 650B: This is the way I do my builds for 650B. I wait and wait and wait until a nice late 80's/early 90's steel road frame in my size comes up. Then I wait for a good used set of wheels. Then I scrounge bits from the volunteer shop I work at, but I'll have to buy long reach brakes. This process has taken (for my recent build) more than a year. I do this to maximize my $ savings. If I wanted to get it done fast, (buy a custom wheelset, new bits and pieces, and a higher priced used frame) I reckon I'd spend at leaset $600+ bucks - near as much as, say, a used Trek WSD 1000 or 1500. On the other hand, when I get my 650B bike set up, it's fun as hell, unique, and my pride and joy.