The basic peripherals on megaAVR microcontrollers are common throughout the family. In addition, some specific peripherals like LCD controller and USB are available on dedicated devices. No one likes to use a microcontroller that they do not fully utilize. If the requirement is two USARTs then pick the device with two USARTs. There is no need to buy a device with five USARTs and leave three of them unused. The wide selection of megaAVR devices allows for this flexibility. The requirements list from the start of this presentation can be used to illustrate the wide range of megaAVR devices. Based on the list it looks like the ATmega168P could be a starting point as it is a good fit for this ultra-low power application.