This is pretty much in line with my philosophy of functionality first then infrastructure. The problems that inevitably sprout while building the functionality will influence future infrastructure / plumbing decisions. Of course, some care has to be taken to not entangle everything, but this shouldn't be too hard if you take a simple services-oriented architecture or even forego services and just store all your functionality in model classes. Regardless of the approach, the first thing that should be written are the tests that validate the functionality. Getting those passing is the highest priority; how they pass comes after.