They have a disabled person living with them, that they take care of, am I responsible for following ADA codes?

ADA Consultant11.

Service dogs and landlords ,what I understand of the ada that made the laws governing a service animal they can only ask two questions what's a person's disability and what was it trained to perform,and first there's no registration required on the federal law but some states have made laws requiring a service animal to be certified most don't and the federal laws states due to so many are poor that self taught is legal yet they are required to log in on notes of the daily task of training and it also states they must be housebroken and not to be aggressive.other words under control and they can not refuse housing or restrict places but some states have and you have do your own homework or be liable Illinois where I was born and live protect owners of also and do not require a certificate of a professionally trained animal to be considered a service dog .

Definitely do a test ad as @Reid Hanley suggests.

There are Zoning, ADA, Safety, etc.. laws that may impact you. 

Read ADA.

These will be less expensive options but be aware that the municipality might be asking for more.If the municipality is asking for zoning issues, ADA requirements, historical items or design changes, etc. then get an Architect.

I'd ad a nice striking chandelier ( not white) with multiple bulbs new table, similar to what's there now. 

One of our homes will become a rental/Airbnb property and the second home that we are currently building will be our main residence and have an ADA-inspired in-law suite for my parents.  

However, if you really like this house, you can use the wrong ad as an leverage to negotiate to lower the purchase price.