So, to calculate your returns, you're going to need a few items. (1) Total property price (including fees/closing/reno), (2) your initial cash outlay, (3) estimated income (rent), and (4) estimated expenses. For a first glance, pro forma numbers (using the 50% rule and/or seller provided numbers) should be fine. If it looks like you want to pursue it, then you go deeper.
Important metrics for analysis:
Net Operating Income: Yearly Income (total rent) - Yearly Expenses. Using your numbers, this is
7200 - 3600 = $3,600/year
Financing does not change this number. Only income/expenses do.
Cash Flow: Monthly Gross Income - Debt Servicing. If you pay cash, your cash flow = your NOI.
$300/mo
Cap Rate: The ratio of NOI over capital costs for the property. So NOI divided by the total property cost (purchase + reno) If I assume you don't finance, your closing costs will be low, but let's just say you're all in for 32k. (question for the more experienced REIs -- do I factor soft costs into the total property price for cap rate, or just for COC return?)
3600/32000 = 11.2% <-- this number is good, I'd say. It probably also indicated you're in a rougher area, which can make landlording more expensive.
And, finally,
Cash on Cash Return: The ratio of your yearly profit over your total cash outlay. Since you're paying cash for everything, this is the same as your cap rate. While 11% is a sweet cap rate, it's a less sweet COC return. How do you increase this number? By using less of your own money, aka financing. This would mean lower cashflow, though, but better returns. It's somewhat of a philosophical decision, since how much you want to use leverage is directly related to your risk tolerance.
As for whether I'd invest in rentals while I have a bunch of outstanding debt, I can't really advise you there. Good luck!