I recently decorated our beach condo on a budget. Items to consider are bowls or clear glass vases with found shells displayed. Sisal rug and woven baskets make a coastal feel. The baskets are great for storage as well. I used white light and linen drapes found at ikea.Because of small size of room, i have glass tables to help maximze space. I am adding shutters found at a flea market as headboard for guest bedroom this year. I found many useful items from community sales. I frequent local beach decor shops some pricey to get ideas for arranging and adding items. I would put couch so can look out window to garden. Put it floating in the center of the room, Tv on wall. Two chairs sort of where they are now in photo. Small table and chairs located near door and kitchen. Looks like not much space there but look at ikea furniture as it is for small spaces. One ikea table is shown to be against wall and then pulls out to enlarge, it might work for you.

If the roof in the sunroom is also flat on the exterior part of the roof then perhaps put triangular windows in the top part of the wall that would look straight outside and then keep the sunroom ceiling flat. You wont notice its different as the window to the outside will be the feature of that wall.

Removing the walls as you've suggested should be ok as long as you check will an engineer first. They may be load bearing. This isn't a major issue to remove them as builders do this type of work regularly but definitely seek professional structural engineering advice first. Privacy to entry will be affected but not too bad. I'd do a furniture layout first and foremost to see if it the rooms will work being so open plan (i.e. noisy perhaps, where to put lounges and dining tables, etc.). People like open plan to look at but they are noisy in many cases as even talking noise travels. Even worse if you have children playing on one side of the area and you're trying to have a conversation with others in another area.

Some dimensions will help. Do you want a tv in the space? How many seats? A partition shelving unit or console backing onto a couch would create a sense of hallway at front door. Definitely need a large rug in living space to define the area and create warmth.