Should I convert den/living area on ground floor into a master?

[David A.]

I'm about to close on a house (my first purchase!!) and I'm not sure what to do with the layout of the first floor, but it definitely has to be reworked. I've attached an image to this post of the first floor layout.

The house is labeled currently as a 4/1.5, although the fourth bedroom is not legal as you have to walk through the third bedroom to get into it. I originally was planning on knocking that wall down and making it into a larger master bedroom, but there's nowhere to put a walk-in due to the door placement and that's the load bearing wall with the 4th bedroom as part of an addition, so the cost to renegotiate that wall seems to not be a good ROI considering the closet space will not match the size of the room.

The bathroom downstairs is a small powder room, but adjacent to a small storage room and a small walkway from the front door to the kitchen. I want to close that walkway and make it a long narrow bathroom, about 12x5, keeping the toilet on one side and building a nice-sized walk-in shower on the other, with a sink and storage between the two. I would just make a door at the center of the room on the ????? side. 

The room that leads into this bathroom is a fourth space on the first floor; there is a kitchen that spills out into a nice-sized living room, with a dining room on the other end. This room currently has an opening into the living room and at the main entrance, so one of those would definitely be closed off if this were to turn into a bedroom. 

My question is: SHOULD I turn this into a bedroom, or is it better as an ambiguous office? I was thinking about using it as the dining room, but I really need to totally close that north kitchen wall to give the kitchen more counter space, as several very tall windows prevent anything but a 5' stretch on the exterior wall. 

This room is approximately 12x12, which makes it the same size as one bedroom upstairs (the others are 10x12, 10.5x12, and 12x9.5). That would give me the flexibility of turning that 4th not-legal bedroom into an office or whatever and still being able to resell the home as a 4-bedroom, but with a full bath tied into the downstairs master. HOWEVER, this means that the downstairs bathroom is only accessible through the bedroom. I'm not sure if that's a huge problem for resale. Personally, I would prefer to not have a master downstairs, but it seems like 70-80% of buyers are looking for that. I can squeeze a small walk-in closet in here by essentially making the bathroom a few feet smaller, but I wouldn't put in a closet if this weren't converted into a bedroom. 

I'm planning on living in and working on this house for my one-year mortgage commitment. By the end of that year, the plan is to put it on the market and use my profits to start investing in some buy-and-hold multi-families. 

Any suggestions? I realize I'm rambling a lot, but I'm not sure how to better explain this without like sitting down with somebody and pointing at the floorplan haha. You guys are all great!