Given the domain is well known and solution is in use already that can be used to model new solution, if I were in your shoe, I would have started with one dba, and a couple of oracle APEX developers. Oracle APEX, with a solid database in background, can get a lot of development done in a short time. In 2-3 months time, a review of development activity would have provided a decent idea if development should be taken inhouse or left in current state.