If you are going for a flat then try not to get one that is really close to lots of other flats on the landing as the sound proofing can be terrible and your neighbours might be able to hear you opening and closing the door to the clients etc.
Yes, you need to be wary of clients nattering away to you in the shared hallways, too, which is even worse if you have to walk past your neighbours' front doors to get the client into your flat. It's awkward to open the door to someone and have to immediately tell them to shut up while you go past the shared areas, but it's truly amazing how much can be heard by the people around you.
I have upstairs neighbours and I know when they're in, when they're out and when they've got visitors cos I can very annoyingly hear every step anyone takes on the shared stairs outside my door!
I think if I were looking for a new incall, I'd go for a basement flat like the others mentioned above. You don't need to worry about lack of natural light, since the bedroom's likely going to have soft lamps and all that anyway, and if you're not living there it certainly doesn't matter.
A proper spyhole and ideally a video camera entryphone are extremely useful, since sometimes you'll have a client due at 12pm and the postman calling at 11.58am and you want to know which is which!
I'm not sure if busy street vs quieter street matters too much, but think about how you're going to direct clients to your door and whether there's likely to be many men standing about on the street outside yelling your work name down their phone because they're lost! It's best to be giving directions from a nearby landmark/tube station on the phone so you can tell them precisely which doorbell to press if there's a choice; it's amazing how much they can sometimes not listen and I've had at least one client demand my upstairs neighbour let him in.
Try not to ever leave them to their own devices in the building. As I said, keep them on the phone while you direct them to your incall place, meet them at the door (unless it's a very big, nice block with secure doors that close themselves after someone comes in) and walk with them through the corridors so that they don't bump into a random woman and start trying to show her their penis. A client once told me he visited an escort and went into the wrong flat because the neighbour had happened to open her door to put rubbish out or something - he literally went
right into the neighbour's flat and probably scared the wotsit out of her! He laughed about it but obviously he wasn't the one in danger of being evicted due to his mistake. So yes, try not to leave them alone ever - I had someone go through the post that had been delivered during his booking that he found downstairs once, too, and then email me later to tell me I wasn't on the ball about my privacy.
Oh, and I think the most important thing to keep in mind is that wherever you rent, don't go for a contract longer than 6 months unless there's a break clause. You might find the perfect place but a few months in, you could get a stalker, or you could get found out by the neighbours, or you could just hate it for some reason - always keep a 'temporary' feel to things so that you can quickly pack up and move on should a disaster happen.