What you need to do is contact a certified or certified general appraiser to determine the highest and best use of the property and its value as commercial and residential. You also need to check if the property can be rezoned as residential. I get the impression the "old school building" is a stand along single structure or are there additional structures on the property? Other factors that need to be taken into consideration for an appraisal would be the location, lot or site size, square footage of the building(s), the market area, replacement cost, value of the land itself, and renovation/rehab/remodeling costs. As for finding comparables, there are always properties that can be used based on certain search criterias and necessary and appropriate adjustments. After checking on the zoneing and consulting with an appraiser, then you can decide if it is worth it and how much financing you can get if you decide to do it. Most likely you will have to put up more than 20% down.