Ok, I have settled on the MSP-432 with the Kentec display. Out of the box the feature set is quite impressive:
48MHz 32-bit ARM Cortex M4F with Floating Point Unit and DSP acceleration
Power consumption: 95uA/MHz active, and 850nA RTC standby operation
Analog: 24Ch 14-bit differential 1MSPS SAR ADC, Two Comparators
Digital: Advanced Encryption Standard (AES256) Accelerator, CRC, DMA, HW MPY32
Memory: 256KB Flash, 64KB RAM
Timers: 4x16-bit, and 2x32-bit
Communication: Up to 4 I2C, 8 SPI, 4 UART
What else do you need for less than 20 bucks? The price point is lower than an Arduino Mega!
So here is what I need for I/O:
UART 1: Communications to the room controller.
UART 2: Communications to DHT22
A few digital pins for the touch screen.
Now this is written in ANSI-C. So you should use the TI CSS (eclipse), and TI has a huge library of graphics primitives, and I/O library. I don't know is Sketch (Arduino like IDE) supports this board.
Most of the communications is between the DHT22 and the display, with commands like turn on the fans to the room controller. During various times a user will adjust the temperature or the humidity so this must come first, or as the highest priority for an interrupt.
There is also commands going to the thermostat from the room controller. For instance every minute the clock is updated by the room controller, from the network clock.