Real Estate here in Silver Spring has really gone up a ton in the last ten years. MoCo as a whole is really expensive, so 300K, while It might be average for a starter home in the US doesn't get you started here.
For a primary residence, my advice is to keep that a life decision and not an investment decision. (I know this goes somewhat against the grain here on BP). If you want to live in a certain neighborhood, then do it. Don't live somewhere just because you think it will appreciate, or you can house hack, if you're going to hate the area, hate your commute, and make yourself miserable.
As for telling your realtor - just be straightforward. I'm sure you're not the first person to change your mind, and your realtor would rather know sooner rather than later so they don't keep wasting their time. Just call (or email if you're too non-confrontational) and say that based on your current situation and the state of the housing market, you've changed your mind and don't want to buy a home right now. Thank them for their time and say you'll be in touch if/when things change and you decide to try again.
I invest in both PG county and Baltimore county. As @Russell Brazil mentioned above, Baltimore is a very different market. It will cash flow much better than DC, but the city isn't as good. The schools aren't as good, the municipal services aren't as good, the demand is lower, rents are lower, expect less appreciation. Urban blight in Baltimore is far worse than DC. Baltimore city and county both require rental licenses and rental inspections but they're pretty basic - hot water, smoke alarms, everything works, railings on stairs, etc. There is also a Maryland-wide lead inspection that is required for any property built before 1978, but that applies anywhere in MD, not just Baltimore.
There is a very active investor community in Baltimore that I've found invaluable (especially as an outsider trying to learn the city). Search meetup.com for BWI meetup and search facebook for local REI groups.