As a retired builder, I can say this: I have never seen a 1950's rambler built in Arlington County that could not be made to support a second story. A lot of "pop-tops" are being built in Vienna, too. That's just a simple engineering problem. Even foundation work, with modern engineered materials and techniques, is not that big a deal.The design,in and of itself, represents a considerable investment of time and money. I don't think that a seller in this heating-up market will be willing to take his property off the market for the two months it would take to find a builder and have his architects and structural engineer prepare a design. Furthermore, under the new statewide building code, building permits expire after, I think, two years, if work has not been started.
I would be much more concerned with saving the money to do the "pop-top" right, which, currently, is running at least $250,000.00. You will want, at a minimum, to raise the ceiling joists up so that you have a high ceiling. I think you should buy the house and look at a lot of "pop-tops" in Arlington and Vienna. A lot of the ones in Vienna, especially, are really attractive. Your Realtor should be able to put you in touch with a reputable builder, or, if not, you can contact me for more information.
Typically, you don't want to spend time or money on a home until you have it under contract because if its not under contract, anyone could buy it while you are spending your money on inspections and due diligence. So, contracts can be written with certain clauses to give you control and protect your interests. For example, "Subject to confirmation that a second story addition can be added to home," or something similar to that. You can also call the county and ask some basic questions. Even easier would be to look around the neighborhood and see if any other homes have had second stories put on them or if homes have been knocked down and rebuilt with two story homes.
I'd be happy to help you if you would like. I am an agent who specializes in helping clients buy and renovate homes as either primary residences or investment properties.