Can try to reposition to Class B, but neighborhood may impede these efforts.Vacancy Est: Historically 10%, but 15-20% should be used to also cover tenant nonpayment, eviction costs & damages.Tenant Pool: majority will have FICO scores of 560-620, many blemishes, but should have no evictions in last 2 years.

All that does is cost you more money. 

Along with many others I'm sure, I read "The Book on Estimating Rehab Costs" by @J.

I'm afraid skylights might leak or increase the cost. 

Can try to reposition to Class B, but neighborhood may impede these efforts.Vacancy Est: Historically 10%, but 15-20% should be used to also cover tenant nonpayment, eviction costs & damages.Tenant Pool: majority will have FICO scores of 560-620, many blemishes, but should have no evictions in last 2 years.

You simply add up the “net sale price” (after removing selling COSTS, the mortgage isn’t a cost.) that how much you have to buy to avoid paying any taxes.

Cash-on-cash returns are what remain after vacancy costs, mortgage, and expenses.

Down payments are still standard for FHA or conventional (3.5% of the total cost, purchase+construction) and rates are on par with standard FHA rates. 

We have substantial closing costs on the sale, primarily transfer tax and realtors' commissions.

Usually after walking through the property he is able to give me a good ball park figure of what the renovation should cost barring unforeseen issues.