I have a Monzo card that's not related to my main account and always transfer spending money there. They don't charge fees and I really like their app in terms of how you can see easily what you've spent and where you've spent it.
We went in December and found the train, then subway easy from the airport (there are lots of staff at the ticket machines at JFK if you're tired and just need a bit of help).
We booked hotel through booking.com for free cancellation and picked one of the cheapest in the area we wanted and main pre requisite was that it had private bathroom, 24 hour desk, near a subway. It was loads nicer than the photos.
We actually wanted to be in a touristy area for convenience. It was DH first visit so we wanted to do all the obvious stuff and soak up the NYE and Christmas atmosphere but I'd happily stay in most areas of Manhattan as well as Brooklyn, depending on what you want to see and times of day.
Although Manhattan is relatively small, you can be slowed down by crowds, constant traffic lights if on foot. Subway is fine, not as slick as London underground on terms of information, entrance and exits and the trains didn't seem quite as frequent.
Have a great time, we loved it!!