Previous owner added a mother in law suite with separate entrance to the property and was renting it out.

In my opinion, changing locks would not be necessary unless there is a vacant unit, or possibly if there is a common main entrance that all tenants enter through, I might change that and provide new keys to tenants as soon as they complete a new application for your records.

We do smart keys without the control key on common building entrances and we use keypad locks from garage to house doors (to avoid lockouts).

She told me the doorknob to her unit entrance was broken.

Look for properties with space that can be converted into a MIL suite if there's not one listed, like maybe an enclosed carport with separate entrance, or a walk-out basement.

It is a 26'x28' 2 car garage with private door entrance as well.

But, the idea of setting up a SFH with a in-law basement with a separate entrance might work for us.

Critical things to look for as far as establishing continuous operation as a multi-family is that each unit has a separate LG&E meter (also essential to keep costs under control as several others have mentioned), a separate entrance, a separate kitchen, and a separate bath.

We ended up installing a private entrance door and gate to the bedroom[room is in back of our house] which I think is a big plus. 

To mitigate these risks, I recommend the following key management system we have evolved overtime:1st - Secure the entrance of your property: This should be the first line of defense against unauthorized access.2nd -Utilize a digital lock code: This can be installed in a closet if possible, and will provide an additional layer of security.3rd- Install a key-coded key box in the closet: Choose a key box that is secure and not easily accessible, such as one that is bolted to the studs and not installed in drywall.4th- Utilize a key ring with a tracker: Keys can be labeled with coded labels, and a tracker such as an Apple Tag or Tile key chain can be attached to the key ring to help prevent loss.5th - Label individual keys with codes: This will help prevent unauthorized access in the event of theft or a key breach.