Based on the Trinity Study, you could expect to be able to take 4% of your investment (in index funds) and have the principle amount retain its value, keeping up with inflation, theoretically forever.
If you sell for $180k, and walk away with $70k in hand (assuming you cover some closing costs or pay a sales commission. I know you said you were hoping to do FSBO, so you might walk away with more) Invest that in index funds, and using the 4% rule for safe withdrawal rate you could expect to earn $233 per month.
You said you think the property could cash flow for $300 per month? Is that using the 50% rule? (over a long period of time you should expect to spend 50% of your gross rent in costs related to the property. E.g. Maintenance, Management, taxes, insurance, etc)
This is, of course, without taking into account things like market appreciation or payments to the mortgage principle.
I am personally scared to death of being a long distance landlord, but I know there are a lot of people here who are, and they do just fine. My strategy is cash flow, and if and when my properties have enough equity that I can sell them and make the same return through index funds (which are essentially 100% passive) I plan to sell and never have to stress about a broken toilet again. It sounds like you're pretty close to that point.