When I became pregnant with triplets, who are 3 now, we realized that we would need a nanny simply to be able to actually feed our babies, and also look after our older 2 girls who were 2 and 3 when triplets born.
Not all nannies will be sole carers - our certainly isn't. We work as a team in the same way that DH and I would work if he wasn't at work.
I.e, when our triplets were newborn, one of us would 'entertain' my older 2 DD's and the other would care for the babies. 75% of the time I would feed and change babies, but it was also nice for someone else to take over so that I was able to play with DD1 and DD2.
Our nearest family is about 2 hours away - when our DTrips were first born then family would travel up to help, but we couldn't expect them to do this forever.
DTrips were 3 in July, and often I think that they are at the age where we probably don't need an extra pair of hands, but then they will go through another 'phase' and I realize that we definitely do! They are currently going through a phase where they flatly refuse to do absolutely anything - its bloody helpful having an extra pair of hands!
If you definitely decide to employ a nanny then I would do it ASAP so that your daughter gets to know her properly before you give birth. We did the same thing as I was also on bed rest and so it was a perfect time for our nanny to get to know our DD1 and 2. Around the birth of DTrips, our nanny was invaluable - esp when they were in SCBU in hospital as premature. It was a relief to know that they were with someone who they adore and know - I wouldn't have been so happy about leaving them if we'd only known the nanny for a week or so.
Go for it!