and if so how do I calculate that and add it to my tax expenses?  

I do inspections semiannually.In my relatively short tenure as a landlord, tenants that are "up to no good" have both (a) paid rent like clockwork and (b) been unresponsive to any communication.In addition, my lease has the following provision:  (o) You shall keep us informed of your current contact information at all times, including phone numbers, emails, and emergency contacts.

The entire 2 flat project with an addition at the top took me about a year. 

If you are in an area where the market will allow you to collect last month's rent in addition to security and first, I'd get them on a payment plan to come up with the last month's rent in addition to regular rent payments.

I've done this before and it has worked nicely.Other than that, consider whether your lender would allow an additional loan to cover closing costs.

I look forward to connecting with other agents as well in addition to investors.

We inserted a partition wall in the large addition on the back of the property to create a master suite, and a family den.

This is what I'm looking at:https://chicago.craigslist.org/sox/reo/d/mixed-use...Additional pictures of inside: http://www.inexpensiveproperties.com/index.php?

The home is deemed inhabitable due to the unpermitted garage addition, but there are currently tenants living there illegally. 

I agree with most comments above, it is very likely you will have the property in your possession again in a few years, the other issue is, lets say you apply $200/mo of the rent to principal, because the tenant is "renting to own" Fannie Mae, Freddie Mac and FHA do not recognize the additional rent paid to principal for down payment, because they lend on the purchase price, and no matter how you word it, they will not accept $10 or $20K as Down payment for the loan if it was paid to you over time. also if you need to remove them from the property it is possible a judge may see the principal payment as equitable interest in the property, there are many hag ups in this strategy.