What is your budget?
Understanding the breadth of your question, I will point out a few things to know about the Nova market, especially the areas in Fairfax county which you have mentioned. McLean and Vienna are some of the priciest markets in the DMV area (McLean especially). Investors are active in both cities, mainly in single family development. Teardowns and rebuilds of old construction are extremely common in those cities and you will see them far more than flips, simply because of the market demand. Buyers in those areas dont really care for a face-lift for the price point that they pay for. They want a new house. Some neighborhoods have a new construction on every street, and another in progress next door. The Town of Vienna has strict regulations and I caution you to do your research and really understand local codes before diving into that market especially. A colleague did a complete teardown rebuild 18 months ago, and had to redo the driveway because it was a few inches too wide, costing $10,000+ to redo. Because of this, you see the same handful of investors dealing in Vienna.
Fairfax is similar in some parts, but not as active for flipping or teardowns because there is a lot of new construction from larger developers in Fairfax currently, especially in Merrifield and Fairfax Circle.
As for the rest of Nova, flipping is prominent all over, it just depends on where you can make the numbers work. I have recently seen several flips in Alexandria and Manassas, but it really depends on where your numbers can work.
Many buy-and-hold investors in this area play for appreciation (for good or bad). It is possible to find a cash-flowing rental property, but it is difficult and takes dedication, and time (depends on your price point of course).
To your strategy of multi-families, Northern Va is almost entirely empty of this type of unit. For several reasons, including tax codes, urban sprawl, etc. You would need to go as far south as Richmond, or as far north as Baltimore to really find consistent inventory of this type of multifamily, though parts of DC and Maryland will have some supply.