i need to create an appointment letter which will be saved in a word template and user will enter the details in infopath form and while submitting it has to take the inputs from the infopath form and save as word or pdf file from the template i saved
Store that data in a database.
And then later you want to use a mail merge to generate letters to the appointment holders.
It could be done with the tools you describe but to make this simple, use SharePoint.
Create a List that will contain the name, email, date, times, whatever. you can customize the input form to see if you can prevent double bookings, define the allowable appointment slots in another list, etc.
You also could create a special account and outlook and then have all of this out of the box and enter appointment holders in as contacts.
But then just create your word letter and connect to the SP list for the mail merge.