I live in VA about 20 miles S of DC on I-95. If you think you might only be in DC for 3-5 years, I would rent. Because the market cooled last year, there are TONS of really nice houses for rent. You can easily get a 3 year rental contract. And the rents DO NOT match the prices of the mortgages if you bought!
For ex., in an upscale golf course community, you can rent a 3500 sq ft 4 BR home with granite, etc. for about $2500 a month. The house sells for $800K now and used to sell for $900K 18 months ago.
In the $2000 a month range, you can still get a very nice house in a good school district in my area (eastern prince william county, va)---but for $2500 you can live like a KING.
Search 22192 for rentals--also in woodbridge/lake ridge area. (IF you are interested, I can give you the low-down on schools---there is a difference, even within this zip code.)
$300K will not buy you much at all---townhouse, if that. I would rent and live in a great neighborhood with good schools unless you KNEW 100% that you werew not moving for at least 5+ years.