Thanks for the response.
As far as connected hardware all I will have is a single analog I/P and 3 digital O/Ps to switch 3 transistors.
I will also be having a colour 1.3" OLED display that will display a single digit (out of a possible 7) as a bitmap that scrolls left or right /up or down (depending on the orientation of the display). This will only change with as the level input on the analog I/P changes up and down.
With all that I don't need huge I/O and speed doesn't have to super fast. I am thinking 10 - 16Mhz should be ample. I was thinking the 3216/3217 would be suitable.
Having never done this sort of thing before is why I am asking for advice from the experts.