Being a good investor is like any good habit. 

Wow I really admire you for taking this on and bringing it back to where it is habitable...avocado green appliances are back in aren't they :) 

. * Deducted $100 in rent twice saying food was spoiled due to refrigerator issues (fixed promptly)* Rejected the contractors being sent by home warranty company thrice saying they were rude and asked for alternate contractors* Told me that they found black widow spiders and were concerned that  the pests could harm their kid .

Welcome aboard Adam - I've just started making BP a daily habit. 

If you do decide to keep them, raise the rent slightly (unless you already have, and/or are at market for the condition the apt was in when they moved in), get the repairs done to insure habitability, and give them a deadline and a cost for replacing the AC filter ('replace AC filter by x date or we will do it and charge you $y as added rent' assuming your lease allows such).  

As far as evaluating properties, these links will get you started. https://www.biggerpockets.com/renewsblog/2014/02/14/70-rule-bible/https://www.biggerpockets.com/renewsblog/2010/06/30/introduction-to-real-estate-analysis-investing/https://www.biggerpockets.com/renewsblog/2013/01/19/real-estate-math/Make a habit of searching the forums for things you want info on.

Additionally, the legal system is set up to favor owners of storage facilities as opposed to being "tenant friendly" like it is in the world of habitational real estate.  

Bad credit means a habit of not paying bills and poor financial judgement.

Calculate how much extra this tenant's habit is costing you and factor it into the rent or add an extra charge for excess utility use.

So it may be a little late for this post, as I am already under contract but I figured what'd be the harm in asking right?