HOW DO YOU DETERMINE THE REPAIR\REHAB COSTS OF THE PROPERTY FROM THAT YOU CAN NOT PHYSICALLY SEE.
It is very hard to know exactly what's in the structure, with that said knowing the age and the area help you understand what your facing. Try to see and evaluate as many properties as possible in your target markets, try to get a general contractor on your team who can meet you and go through a few properties so you can understand the issues your facing.
If your good calculating square footage and linier feet you can acquire at Home Depot or Lowe’s “Construction Cost Data” books by RS Means. But experience is probable the best and you will need time to develop that so don’t give up.