What factors that affect valuation/appraisal values of SF homes?

[Simpson St Louis]

I am new to investing(wholesaling), so one could expect that i wont have all the right answers. 

Now moving on my dilemma is more than likely a simple one, but I don't want to waste anyone's time neither seller or buyer or my own. So this is the scenario I  find a property in distress. I go about  the regular process of taking pics write notes of what see and notice.  I see the property has some boarded windows some are broken, looking inside I see moderate damage to the walls and cabinets the floor are tiles, etc. So after seeing all this Im thinking that this house which is a 3/2 in this condition is probably worth about 40-55k roughly. I know about the 70/20 rule but is that alone able to justify the rough estimate of 40-55k purely off what I see, and feel that is itis value. Or are there more factors that need or should be accounted for before contacting either the owner or a buyer?