I have lived in the Sandy Springs - Dunwoody area for nearly 23 years and used to work Downtown at the Apparel Mart AND I commuted to and from the airport nearly every week for over 2 years. If you purchase a home in Dunwoody near the Village, you could take the Sandy Springs Marta or the North Springs Marta since both locations are next to Dunwoody and have lots of available parking at the stations. The Dunwoody Marta Station is actually next to Perimeter Mall and the area is congested especially at the end of a work day. I wouldn't even consider using the Medical Center location; not sure why that was offered. From the Dunwoody Village area you could hop on GA400 or go a little farther south and get onto I285 directly to I85 or GA400. Commute times will vary based on traffic and construction anywhere from less than 30 minutes (at 5AM) to as much as over an hour.
Just remember that Marta trains are NOT fast. If you moved to Brookhaven you could take Marta from the Brookhaven / Chamblee station which could get you DT a little quicker. Also from Brookhaven you could hop on I85 and get DT in about 20 minutes....but you are going to pay quite a bit more for a home in Brookhaven than you will in Dunwoody (depending on the neighborhood)....... more