This asks more questions than provides answers:
It's clearly a simple logic function which holds the o/p at just below 0V (high) when the control switch is inactive also at 0V (high). Any signal present at the i/p will be overridden in this state.
The circuit therefore represents a tri-state buffer with active low control:
When the control switch is held at the -ve supply rail -15V (low), the o/p logic follows the i/p logic signal.
So the important questions before calculations can be made and components chosen are:
1) what are the switching threshold voltages? i.e. how critical are they and how close do they need to be to the 0V and -15V rails?
2) how much drive current can be taken from the i/p signal to activate the first transistor? Are there other components hanging off the preceding drive stage (i/p to this stage) which needs consideration?
3) how much current does the o/p sink to the next stage? i.e. what is the loading on the o/p device?
4) Is the switching speed a critical parameter? i.e. rise and fall times and switching frequency?
5) Are you sure about the logic voltages? +15v will cause a reverse bias across the PNP devices base-emitter junction and hence exceed it's rated breakdown voltage which is likely to destroy the device.
6) Does the operating environment need consideration? i.e. what environment does this need to operate within?
7) Is this a one-off proof-of-concept project or is this something that needs to be taken through to production? i.e. is cost a factor in the engineering?