For those who are wanting a nanny for the first time, I would really suggest using an Agency, unless you know a lot about recruitment, employment law and tax rules though your current job. Agencies do charge a lot, but a good agency will advise you on the many matters concerned with having an employee in your home.
If you go it alone, then as a nanny, I mostly look for jobs (when I need to) over at NannyJob where parents can advertise for a fee (about £10) and search for candidates for FREE. The message board is also worth a read at times - as you can find out what issues come up between nannies and employers, though no advertising as such on there.
There is an sample contract on the site as well which will help if you are not using an agency.
Time is passing by fast. Nannies who are in a job will have a notice period, a month is not untypical. Obviously easier if the nanny is out of work.
The bond between a child and their primary carer can be very strong. It does not surprise me that your eldest child called his nanny Mummy. I am dreading the youngest I care for (currently 17 months) saying what he wants to call be for the first time. His older sister used to call me "Nanna Nee", but has recently been able to say Nick - which I prefer. I used to care for a 4 year old, who choose to call me Daddy when it suited him (such as when I picked him up from pre-school), I suppose it was more socially acceptable to be collected by Daddy than your male nanny!
As your husband is 'old fashioned' guess he won't consider a male nanny, on principle. Not that I'm looking for a job, maybe another year perhaps!
Fitting in with the SAHM lot can be very difficult. I don't even bother now, though it can affect the social life of the children I care for. Luckly a good friend of mine is a childminder, so we often go to groups together so that at least I get to talk to someone! I do cook organic food, but I certainly don't dress up for the school run, avoid coffee mornings like the plague and go to a friendly toddler group, which has taken ages to find - most of the groups local to me were not that friendly!
Have a good balance to help with guilt feelings. You need to work to keep your mind active, to keep up with your career, and to feed, clothe and home your children. So don't feel too guilty - just aim to get a good balance, which means that when you are at home, you spend quality time with the children, doing children type things - not dragging them around an art gallery!
Good luck finding a nanny.