My first thought...is there a medical reason you need a night nurse? If so, an an pair will not be able to provide this level of care.
If it is just for your sanity, then I would recommend the famous baby training book or CD, where the English nanny teaches a new mom how to train her babies to sleep through the night.
I have lots of friends with twins with siblings and they have done very well managing with far less help. They all seem to rave about "Mothers of Multiple' clubs too. Especially with the age spread you have, your 6 year old is reasonable and mature compared to a 2 year old.
So, I have had both types of help....Nanny's and Au Pairs. I did not go through an agency for the au pairs, so did not pay a fortune. I speak a lot of German so I was able to find wonderful German girls. Both have become life long friends to me and my kids. The pay was very little, as there is an exchange of room and board and time off for her services. It was nothing rigid, as the girls essentially became part of the family. We enjoyed each others company so much, that they accompanied us on vacations, etc. But I invested a lot of time in teaching them English and taking them out and about.
I was also concerned about having someone in my home 24/7, that I would not get my personal space, which I need tons of. It was an adjustment in the beginning, but at the time, my husband was traveling so much it was beneficial for us all having another adult around. As I don't have any family that helps. I have family that drains.
Ditto to the gals below also.