Also, I would have to factor PMI? 

Here are the details: Duplex built in 1999Target Purchase Price: $392,000Unit 1: 4 bedroom 1.5 bath rents for $2100/monthUnit 2: 3 bedroom 1.5 rents for $1900/month (we would occupy this side)Total monthly income: $4000Expenses:Principle + Interest: $1950Insurance: $100Mortgage insurance: $253Taxes: $720Vacancy (5%): 200Maintenance/repairs: $330 (1% home value/year)Cap Ex (5%): $200Tenant responsible for utilitiesTotal expenses: $3753 (this does not factor in property management as we will be owner occupying, however if I factor in property management at 9%, it would add $360 which would be negative cash flow).Total cash flow $257CAP rate: 6.4%Cash on Cash return: 9%Without property management we should have positive cash flow, given that I am estimated expenses correctly. 

What are the factors that should be considered?

The #1 factor in making money in class d/f areas is market knowledge and knowledge of how to deal with the issues that arise from investing in a high risk area.I grew up in a class d/f area, and Ive invested in those areas in the past.

I have an opportunity to get a SFR for 45k that is in a rough area but newly renovated with new hvac, water heater, and roof. rent should be 750 per month, taxes 650, ins 900. assuming 35% for capex and repairs and 10% for PM. assuming 8% vacancy factor and 5% leasing.

The hands on approach will help you and ease the fear factor in stepping in and doing your first deal.  

Dear BP,I am interested in buying in PR due to many microeconomic factors that make it feel like a good time to buy.

You are correct, I didn't factor in vacancy rate, repair costs etc so that will bring the expense % up.

All depends on multiple factors.

I think a large factor has to do with the age and condition of homes.