Since your only round the courner I would recommend that you simply give me the facelift bumper (for free) and sell the car as original.
But to be honest, it all depends on who's buying the car, as some people will like the new bumper but some may be put off (god knows why)
Edit: Why are you selling ! If you remap then your around 180bhp, what petrol where you thinking off?? As that BHP with the torque + MPG and tax etc is hard to beat