I'm just outside of the upstate in Rock Hill. I've gotten my seats out and put the flooring in, everything else is stock (ceiling, walls, etc).
If you're not too far in the mountains (don't want to test the 545 with mountains just yet) I can ride up and you can see what works for you in terms of aisle location. I've got 72" headroom (the absolute minimum), and at 5' 10" I've set my max aisle location to just under the stock lighting running down the bus, about 1 1/2 feet from the wall. Any closer and I start to hit my head on things.
If you're 5' 1" you won't have any problem with a wall aisle on a low height bus.
While it may be annoying for taller friends/family, wall aisles allow for a much larger rooms (if you decide to go that route) and if not, then much larger open spaces that are not interrupted by thoroughfares.
One thing I'd suggest in the floorplan stage - take a look at trains, specifically passenger trains. They have minimal space as well, and some I've seen have gotten creative with their layouts. Especially the bedrooms and lounges as they have a moderate amount of "reconfiguration" - ie the bed can turn into a chair and table.
Another suggesting is making floorplans out the wazoo. Always nice to have multiple options/references, since Murphy will always make sure something doesn't fit "just" right. Right now I'm likely going to implement a floorplan I had for a TC2000 transit for my dognose.
Dorian Proctor on Sketchup Warehouse has some amazing and accurate sketchup models that I used. You have to scale them down, but once that is done they are nearly 100% accurate in everything - windows, wheel wells, doors, etc.