I wouldn't worry too much about it being perfect - the email you got isn't perfect either. I would say: Thank you very much for inviting me and giving me this opportunity to meet you. I would be happy to come this Monday. Please let me know what time is convenient for you. 'If possible' is not necessary, as they have already said that it is. 'In your free time' usually refers to time out of work, so isn't quite correct. If you want a few more words, you could say something like 'Monday, if that is still suitable'.