Ok - this is my opinion ...
YANBU to want a break. Babies can be hard work!. I haven't been in your situation as I did have family around me and although I didn't rely on them much, I only had to ask and they would help (I also have 3 dc).
Personally speaking, I really, strongly don't believe that a nursery is the right environment for a baby so young. For a start you would be really hard pressed to find a nursery to even accept a baby so young.
I would worry about the 1:1 attention that baby would get.
I would worry about baby's susceptibility to picking up illnesses.
I would worry that needs are not met immediately.
I would worry about the lack of a 'home' environment.
These are my worries, not yours, I am just giving you a different perspective.
My dd2 went to nursery one day a week when she was only 3 months old (needed childcare for work). I took her out when she was only 5 months old. I hated it.
She picked up virus after virus and although it was a lovely lovely nursery, I just couldn't shake off the feeling that it wasn't right for her, or me. Again, my feelings only.
I would urge you to think about a different way of getting that much-needed time. Meeting some childminders and getting to know them would be a great start.
Oh, and I think its great that you are thinking ahead and taking steps now. Good luck.