While it's tough to buy in SF for this price threshold, it's certainly possible. As an investment, I would steer you toward studios in better neighborhoods with better finishes. There are a few possibilities in this category available in the city right now.
There are one-bedroom and two-bedroom spaces available in this price range, but they will be in less desirable neighborhoods and/or they'll be in rough shape.
Another option would be the East Bay... There are several options that come to mind - both new developments and resales. Within your $300,000 price range, there is at least one development offering over 1,000 square feet, two bedrooms and views.
I would love to discuss these ideas with you in greater detail. My leasing partner can furnish rental rates.
Take care and good luck!
Urban Bay Properties, a division of McGuire Real Estate