The SiLabs parts the Erik crows about would seem sensible to me

I do not, specifically, recommend the SILabs parts they are quite expensive. That said, I use them extensively because of the speed, but if you do not need that, or the advanced analog I/O then $30 (1 off, the price at 10.000 is quite lower) for a '51 is quite stiff.

However, I recommend strongly the SILabs devboards because they come with software that gives you an ICElike development environment and THAT is a find for the price of the devboard ($99-up).

Now with the 2 statements above taken together, if you are only going to make a few units, there is nothing like the SILabs chips, you save so much time developing that $30 for the chip is dirt cheap.

My opinion: if you buy a development board that does not come with ICElike functionality (I believe thare are others) you made a mistake.

Now, if you are in an industrial setting with a reasonable development budget for a large volume development , I would ignore all the above, find the best processor for the job and buy a matching ICE (I have 7)

Erik