Hi - You can use the pins as I/O if you do not use a crystal, You can set the chip to use an internal 8Mhz (or 1Mhz with divide by 8 set). OR run the chip at upto 20Mhz with the crystal & caps. Having built over 100 projects with 45/85 chips, I have not used a crystal setup yet (other than on my programming board, just incase I set the fuses to use external clock).
Yes... however in my case I need a lot of power because I have to make 3 FFT from 3 microphones continuously... so 20MHz better than 8. But I do not have pins enough if I use a external crystal.
I need 3 analogs input for the mics and one pin for sending the information to another Arduino.