If it were my space, I'd do the tower concept with careful planning of stuff to be stored -- sort by height and frequency of access. Drawers are good - especially if they're on full-extension slides. And high shelves are pretty useless for short people...

For a two-bowl vanity, it's 15" minimum between the centerlines of the two sinks, as well 15" from the centerline of each sink to the adjacent wall on either side of each sink. That's an IRC code minimum. Those specific numbers probably won't be found in the actual plumbing code, as the plumbing code is more geared towards the actual plumbing and not the layout. IRC, or the actual building code, is the code that contains minimum space requirements. As far as code hierarchy, IRC is for the most part the building code that covers the United States as a whole. Individual states then have their state codes. Most states simply embrace IRC as a whole, though some states add some additional restrictions geared towards that state. Example, a state like California may have added sections that address seismic concerns. Then there is local code. Individual towns or cities may have additional restrictions dealing with their very localized issues. Historical neighborhoods, etc. For the most part, local and states codes can't relax national (IRC) code requirements, but they can be more restrictive. Sometimes code is added for political reasons. Example, the plumbers union in Massachusetts has a very strong influence on code there. Same with electricians and the electrical code in Chicago. Restrictions are added to try to make it difficult for DIYers to do any work on their own houses.

Naween, I am trying to make my 5x8 bathroom with a shower/bath combo off to the side into an accessible bathroom "just in case", so I know a bit about what you are going through. I am still in the design phase. My issue is that the entry door is on the 5 ft wall, so if I make it a 36" door, as I must, I end up with an 18" wide vanity space. Given that issue, I see the utility of Kraftmaid 2, somewhat, I think it is a 12" deep vanity base for a overhanging one-piece ceramic sink top. The front is there for when no knee access is needed because no chair user is using the sink. It is removable for when knee access is needed. Since the vanity is 27" high, maybe it is a thick china Eurosink like one by Duravit, and it does not over hang the vanity, giving the need for knee space for a chair user to use it. Maybe I am wrong and I am thinking this way because this is the issue I am having designing my bathroom - how to get the most utility out of it while I can still walk, while making it easily convertible to a wheelchair-accessible space should the need ever arise.

My master bath re-do began with a double vanity / sink combo in the bedroom with a toilet and walk in shower in a separate room off to one side and a gigantic roman tub in a separate (but not closed off with a door) area to the other side. The vanity was one of those oak front particle board pieces of garbage with a cultured marble double sink and one of those gigantic mirrors overall. The walk in shower (which began the whole re-do) leaked underneath (inaccessible, of course) and into the kitchen below. So. Now I have a toilet and single wall sink with cart instead of vanity in the room where the toilet and walk in shower was. The roman tub (which was too big to be filled by our standard sized hot water heater) is replaced with a freestanding tub and shower combo (shower curtain all around) with easy-access plumbing (i.e. easy to spot leaks). In the center area that is part of the bedroom is just a freestanding mirror and a hanging plant under the skylight. Very minimal, clean, private. There is a little "ledge" under the free-standing floor mirror covering the plumbing that ran across the floor to the tub. This is reflected also by the ledge in the bathtub/shower area that was always there. It also prevents bashing into the unframed but beveled mirror with the vacuum cleaner. I know this is a lot to picture so I will try to come back with photos later. We are still doing some painting. It always seemed icky to me to have sinks in the bedroom and stupid to have a shower separate from the tub. Capping the lines from the double vanity and converting the shower plumbing to sink plumbing was fairly quick and easy (took a plumber less than an hour). I would suggest you hire a professional for the plumbing and do the rest of the job yourselves if you are wanting to go the diy route.