The PIC doesn't really need a clock (it depends on which one you use).

Since I've got them, you may as well use 12F683 - it doesn't need an external crystal or clock (it's got an 8MHz clock built-in). It's 8-pin: 2 power, 1 input and 5 I/O.

If you're referring to replacing the 7415, then I would keep it as it is - you'd need to monitor the lines, and there would be a delay. But then, if you're not clocking the circuit very fast, then it would probably be okay. But, as you mention, it does save on code complexity

For the PIC, you can use any of the output pins (pins 2, 3, 5, 6 and 7) for the VREF generation signals, but use one of the analogue pins (5, 6 and 7) with a trimmer (between VCC and GND) to set the delay.