The midtown tenement walk up provider apartment is a pretty well-established venue. I've been hobbying on and off for more than 20 years and I've seen many mid-market providers in these types of flats. The entrances are nondescript, at times even a bit seedy looking, but that's just par for the course.
Regarding doormen, there are differing views on this. On the one hand, some hobbyists feel uncomfortable with a doorman (usually the provider "tips" the doorman to look the other way). Personally, I feel safer in a doorman building. Buildings with doormen also tend to be better kept and more upscale, so if that's the impression you're trying to give your clients, then it may be the way to go.