The KL2
MKL25Z128VFT4 M0+ 48 MHz 48-QFN 128 kB 16 kB USB-Reg, IDlooks like a winner based on current information. It certainly doesn't hurt that it costs > $2.00
LESS than competing MCUs. Also, it has the better low-power consumption spec.
Unrelated [to MCU selection] question in a separate email.
I think you are on to something with the password safe idea. Not a mega hit,
but a nice little niche that is do-able, and just might catch on. As in Kickstarter
is not out of the question, given a lot more work on identifying WHY people need it
and might be motivated to invest money. If you can make money with a retail price
NTE $50 it can work.
Ron K Jeffries