@Tyler Holowach
1- I can't help you with which neighborhood. There are too many factors to consider there.
2- Short term rental will likely give you the most income. Since you live in the home you shouldn't have much problem with the cities STR restrictions but I do believe you have to get a permit from the city now. Parking is a big issue and STR's are more work for you. Also, keep in mind that it can be quite challenging to convert any part of a residence to a second living quarter. If the city see's you're putting in another sink and stove they might put the breaks on your project and make you jump through hoops. When you're looking for a place it would be a good idea to find one that already has a second kitchen or kitchenette. Another thing to consider is that your mortgage company is probably not going to allow you to count the income from the second unit when you buy so you'll need to qualify for the loan based on your income alone. The first thing you should do is talk to a lender and see what you qualify for if you haven't already.
3- There are a ton of strategies for finding off market properties. The problem is they take a lot of time and money. Most people don't see it through. If you spend thousands of dollars to put out mailers then you'd better answer your phone when it rings and rush over to any prospective property. 90% of the properties you visit will be no good. The best strategy for a retail buyer looking to buy one home is to hit up the many wholesalers in the area and get on their lists. You can pay more for a property than I can so many of these wholesale properties will work for you. There are plenty of DC wholesalers here on BP.