My 2 cents (even though its only worth 1)
As a hobbyist rental property owner, I've always avoided Norristown because of the extremely high property taxes and the vast number of rental properties there. These taxes have to be figured into your calculations along with purchase price, upkeep and maintenance costs, insurance, vacancies... etc etc.
I have always lived by a simple "1% Rule" which is: $Rent =/> 1% of the purchase price. Based on this rule and Brian's (Mittman + Rehling) median sold price of $155,000, the median rent should be $1,550/month for the area. If the property doesn't meet this simple calculation, you will NOT make money on it in the form of rental income. Obviously this is an over simplified formula, but it has served me well.