Expect to be given a routing through MRB to keep you out of the Dulles arrivals, and then to be vectored to join the approach without going all the way there. On departure, the only way you'll be cleared is direct STILL (not on the L-chart) and then on course from there (like direct EMI and then V3 or V419 going back to Wings). I have no idea what they do if you don't have a GPS, but it's probably ugly for someone headed east -- like direct MRB V3 EMI. And the faster you climb, the faster they turn you east.
Canceling IFR after landing and picking up your clearance are easy on the Potomac Clearance RCO, the frequency for which recently changed -- I see there are now two published, and I don't know if there's any difference. Only problem is you can be stuck for a while on departure if there's inbound traffic -- usual non-towered 1-in/1-out situation.
Also, taking the course and carefully studying the SFRA NOTAMs (including the Leesburg Special Maneuvering Area NOTAM) is a really,
really good idea.
Finally, the folks at Landmark, though expensive, are extremely nice to even the smallest light GA aircraft.
And welcome to PoA and congratulations on your first post here.