Depends on if this is going to be your only practice
I'd highly recommend you have a period when you are both there together so he can introduce you to the patients
Probably a minimum of three months and preferably six months
Also a letter from the retiring dentist to all the patients of the practice and I'd say from experience that this would include everyone who has attended in the last five years. This is easy with a computer generated mailing list and a computer image signature at the bottom.
The retiring dentist should not leave until after the print date on your biggest local phone directory, that way you can have his name appear in the book with yours for another 12 months after he retires. Anyone using the local directories will be looking for HIS name and not yours.
I don't use recall letters as I believe they get lost after a few days. My main recall system is thru Lighthouse 360 which is just fantastic. If you are not a client get on board and mention my name for an extra FREE month of services (This applies to any dentist)
However I'd be inclined in your situation to get some fridge magnets made with the practice name on them and phone number and get them mailed out at the recall due date to all patients that have been seen within the past 24 months and ask the staff who are the best referers and make sure they get a fridge magnet as well.
Finally make sure your SOE Search Engine Optimisation is pushing you to the top of the search list for Google for your practice website CROWN Council have a webinar on this
If you can generate enough new patients to keep you busy during the shared cross over period then you will be smoking it when he retires.
Don't change the staff, don't change the equipment and don't paint the office or change it's outside apperance UNTIL you are so busy you can afford to loose the patients for whom too much change will be just too much
Your best asset in this purchase will be the long term staff who have a long term relationship with the patients , loose them and you will loose a lot of patients