Could you give some advice how to politely inform the potential employer that I would accept the job if they would limit the area of coverage to 2 counties. I do not want to work in third county having had bad experience in that area.

Here, what I came up with:

<< Hello dear Ms.XXXX
Thank you again for speaking with me about the position of XXXXX with XXXXX Homehealth services. I truly enjoyed time we've spend talking about the homecare, which confirmed my desire to return to working in that setting. I hope to work for your company one day when you will need help in Y--- and L---- counties with the exception of C---

thank you