@Jay Hinrichs and @Greg Dickerson, How can one find quality builders who will build for less than $110/sq. ft? 

In my opinion get a quality home inspector and pay them to go through.

I've owned and operated my welding business for 10 years now, and not having quality people in your corner, such as an accountant, can be pretty costly. 

It is unlikely you can build a detached ADU of this size and quality, not doing the work yourself, for $80K. 

- Is there a way to improve quality of the tenants in short term of 1-2years? 

Housing prices (SFH, MF, and apartments) all can vary depending on neighborhood class, size/location, time of year you're trying to fill it, quality of finish out, etc.

I've finally decided to get more active with my RE investing endeavors and make quality connections with people in this community, through meet ups, discussion forums, and mastermind groups. 

I planned on getting a quality home inspection.

Last, the pre-finished hardwood is pretty affordable when you consider the relative cost of lower-quality products and the fact it is pre-finished so, nail it down and your done.

One thing you can do is to check out the contractor's other ADU builds, you can see the quality as well as ask for permit and final approvals of those projects to verify.Good luck!