I've been talking to people and it seems like the smartest idea to make the core of the interior first, i.e everything but set dressing, and then make the exterior around it. I'm anticipating some trouble getting both sections to fit together properly, so this way I can make sure the core of the level is done before I start putting in props.
And on that note, I've started modelling the interior segments..
It looks very bare without the props - barstools, lounge chairs, tables ect. - but it works in engine and is to scale, so I can use it to make sure the exterior lines up properly.
More as it develops. Back to you, studio.