Did this person out a name at the end of his/her e-mail? Or is there a name in the "From" box on the e-mail?
If so, then start Dear xxxxx.
Otherwise, just go straight in:
Thank you for your e-mail suggesting the dates on which you will be able to see me. Any of these dates will suit me. Please let me know which one will be best for you. I will look forward to hearing from you.