Probably the simplest cheap solution for the serial connection is to simply use a MAX232 plus a few capacitors. Then you'll have a fully functional RS232 serial port you can connect to your PC (and program with assuming you've have a bootloader on your chip).
The internal vs. external ADC depends on whether you need the extra resolution vs. the complexity of adding the extra chip, interfacing, etc. Factoring in the noise you'll probably get from the pressure sensors the extra bits might be meaningless.