Rear bumper depending on the make and how easy it is to fit and if it needs no additional work will be under £350. But there is loads of varience. You just need to find one someone else has used. Unfortunatly my recomendation is in Surrey so a bit far for you to travel.
Most wont give you a cost till they have seen the bumper your putting on.
Also, so you need the replacment, very expensive, polystyrene impact bits for under the bumpers? Is your zorst still lined up and not cracked anywhere?
And yes you can get replacement sensors from nissan, call you local nissan garage for a cost.