Here in Michigan, the assessed value is 50% of market value.If something similar applies to this property, $90k assessed value would imply an estimate market value of $180k...

Also, what are the estimated market value and the intended cash-out amount?

I was wonder how to estimate tear down and rebuilt of house.That way I can give my offer to seller. 

If you are concerned about material choice get an estimate from your roofers that lists out the exact materials they are using and then if you want to question them or ask them to change materials you can do that. 

We even had a mason look at it (brick building) who already gave me an estimate on how much some necessary brick work will cost.It is a mixed use, commercial on bottom, and apartments on the 2nd and 3rd level. 

The house I purchased was recently renovated so rehab wasn’t a factor in this deal, but being a STR I wanted to go with higher quality furnishings that would stand up to more abuse. (2 years in and that has paid off so far) But I went about 20% over my estimated budget.

It allows you to play around with average nightly or weekly rent rates and number of weeks rented so you can find your break even or estimated profit. 

Calculating an ARV seems to be an easier task, because I can evaluate the sale prices of similar properties in the area, with respect to size, quality and location.However, estimating rehab costs is difficult.

Different websites estimate between 5% and 10% appreciation in a year!

Won't the estimated value of the rents be calculated towards the monthly mortgage?