OP,
That is a long day, but not shockingly long (you will want to be sure she gets a break somewhere to avoid burnout--maybe allow DC2 a half-hour of tv during school hours or something so she can have a few minutes relatively uninterupted.
We used an agency (Your Child's Nanny) because I just didn't have time to do all my own background stuff.
Take into account overtime and factor in vacation and sick days for the nanny if possible. I think most people do two weeks vacation (one of your choice and one of hers) and 3-5 days sick leave. I don't think it will be impossible to find someone for relatively close to what you pay now, and a nanny can be such a help. I work long hours, too (at one point the nanny was working 65 hours a week) and it has been so helpful to have the kids in our home living a normal routine vs. having to pick them up and squeeze the routine into a brief period at the end of the day. When I get home, the kids' laundry is washed and dried (not always folded, but whatever), their toys are mostly put away, homework is done, they have taken a bath and gotten into pjs, and dinner is ready to eat. All I have to do on work nights is feed them and play until bedtime. SO much easier than trying to check homework, make dinner, do bath and jammies, eat and do bedtime between 6-8.
Our nanny also runs errands (she takes the kids and makes it an adventure), so most of the grocery shopping is taken off my plate, along with random things like pharmacy runs, or restocking on food and diapers at Babysaurus. Not all nannies cook, clean or do errands, so be sure to think about what tasks might be most helpful and discuss with potential hires.