First off, you should probably do the exterior first. For the most part, it's better if you start from the outside and work your way inward. Building the exterior with the interior in mind is the best way to go.
I don't think that's strictly true.
Sometimes it can be worse to build outside-in, since you misjudge the room inside for your interiors.
I think it depends on the builder and their personal view at the time.
Whatever you do, please do not build your exterior walls on the baseplates you have already.
If you do you'll be leaving yourself with 5 blocks room between the outerwall and the bar, which is very tight.
I would recommend you build plate bricks on the perimeter of your baseplates first, at least 2-blocks out. That way you give yourself plenty of room inside.
You can also give yourself an interior wall, which can look different from the exterior wall.
If you were to design that first then you'd have walls that you're happy to look at from the inside and match the theme there.
After that you can build walls, roofing, porticos, balconys and what have you on the outside that perfectly fit in the shape of your interior wall. Just having a good place to build your exterior might bring you inspiration for how to design it.
The outside doesn't necessarilly have to fit the inside in style either. You can have crazy interiors within buildings you'd never expect, so don't be afraid to make the outside a different style.
