Generally, here's the rule I follow with stuff like this: If every house on the block has a garage and you remove it from your house, that's an expected facet of the home that is lost and you will objectively lose value when comparing to another house from a "market value" perspective; especially from an appraiser's point of view. 

If you're a single mom with student loans, a car payment, and living paycheck-to-paycheck, then $20,000 would be devastating and a reserve is critical.

I have cleaned up a number of these deals over the years.. they are all great when they are working and everyone is on the same page but when they are not.. there is extreme risk to the original seller especially in Texas were a bank or lender can get a money judgement on purchase money loan.. out west not so critical original seller just gets their credit trashed if it was not already.. but there is no money judgement available to the lender for a purchase money owner occ loan in first position.

How you operate the asset is critical. 

One of my most critical decisions is to get the RIGHT tenant the first time.

Look at each facet of that business model.  

Regulation and permit or license-ability will be critical.

Every business faces a critical decision-making period when deciding whether to accept or go ahead with a business deal.

I came here for advice, constructive criticisms even. 

Assign weights to each criterion based on its importance to you, such as job growth being more critical than seasonal issues.