Smaller wheels do not mean a smaller bike...
My back got messed up with a work injury and bad rehabilitation and there are times I don't want or can't to swing a leg over a full size frame and don't have to compromise anything when I ride my vintage folders.
The Raleigh 20 fits a lot of your requirements in that it is a simple and bombproof bicycle with a low step over, agile handling, and a very nice ride.... for $400.00 you could buy a nice 20 and afford to upgrade the wheels and brakes as this really makes them much nicer.
My better half rides a Breezer Uptown 8 and an R20 folder... she loves them both.