Yes, you can then use a suitable regulator to take the 24vdc down to what the Arduino needs.
Look at using a small switch mode regulator, rather than a linear type if you can.
ebay is a good place to look and they are very cheap to buy.
You can get small switch mode regs that will handle an input up around 35vdc which will be better.
For your op-amp, just google and you will find many cicruits, or search on the Arduino forum or other electronic forums.
Paul