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? 

Being within a 2 hour drive of San Francisco has me contemplating how the market out here would react if California ever finishes the high speed rail that has been in planning.

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.

You can possibly break this down in the following manner: Year 1: Purchase 1 units; Total Monthly Cash Flow = $200Year 2: Purchase 2 units; Total Monthly Cash Flow = $600Year 3: Purchase 4 units; Total Monthly Cash Flow = $1,400Year 4: Purchase 4 units; Total Monthly Cash Flow = $2,200Year 5: Purchase 4 units; Total Monthly Cash Flow = $3,000Year 6: Purchase 6 units; Total Monthly Cash Flow = $4,200Year 7: Purchase 6 units; Total Monthly Cash Flow = $5,400Year 8: Purchase 6 units; Total Monthly Cash Flow = $6,600Year 9: Purchase 8 units; Total Monthly Cash Flow = $8,200Year 10: Purchase 12 units; Total Monthly Cash Flow = $10,600You can even start purchasing more units early on to speed up the process.

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!

Listen at 1.5X speed if you can handle it to get through more quickly.3) The Book on Rental Property Investing - I know you mentioned wanting to flip, but I suggest starting with this book even for people wanting to flip.