Break down each module and figure out the best method of communications, I.E. Hard-wired, or wireless, 433MHz transmitter / receiver type system. Bluetooth, Zigbee, etc.
Breaking down each module would be for example:
Keypad, 4 x 4 type? Are you going to read the keypad using a matrix I.E. row / column driver? Do you need de-bouncing? If so what is your preferred method? Hardware / software?
Sensors, all the same type? Do you need a contact switch, for example, a pull up connected to a wire which when closed grounds the input, or are you doing this all wireless? Or is it a voltage feedback, in which case you need ADC, how many ADC channels.
You require all of this information to select the PIC appropriate, ideally pick one with a scalable memory footprint, for example, if you chose a 64KB memory chip, if for example you feel you may need / want to expand it in the future, do you need more I/O? If so it may be better to re-design, if you think you may just need more memory for whatever reason chose a PIC which has pin compatible upgrades to 128, 256 or even 512 KB of memory. Of course if it is a one off project it wont matter.
Usually the keypad will arm / disarm the system, and you would normally enter the original code, press hash (pound) or star in a sequence or so and then enter the new code to save it. Normally twice for confirmation, better user feedback especially with the LCD if you do it this way. You don't normally have "backspace", maybe a clear button or hold down '0' for 3 seconds or more.
Alarms are also entry / exit zone and trigger to alert (sound) user definable on times (or at least a range from 10 - 60 second for example).
Just a few tips!
Wilksey