If you're going to be working in Arlington, it will be a lot more convenient to live in VA. I also work in Arlington and lived in DC (Adams Morgan) for awhile. The commute wasn't bad at all because I was very close to Rock Creek Parkway and it was a reverse commute. It took about 15 minutes. However, if I took the Metro, it was closer to 45 due to having to transfer lines.
I live in Alexandria now (right on the Alexandria/Arlington border) and my commute isn't too bad. Driving it's just under 30 minutes and that's due to traffic. If it's not rush hour, it's closer to 10.
If you're looking for a MINI shop in VA, there is Mach V in Sterling and MINI Dynamics (they come to you!).