Start by looking at photos on Houzz, and creating ideabooks of what you like, This will help you focus on ideas. Once you are closer, start to plot down spacing and then we can start to better assist you

I think this piece is quite lovely. It appears that your couch and chairs have dark wood so I would be tempted to stain or paint the buffet to match these items. Changing the large modern handles to smaller antique ones might help to make it less bulky looking and tie in with the other items in your room.. I like the idea of putting a mirror in the upper door, however much as I like the criss cross effect in the sample pictures, adding more detail may conflict with the curves in your sofa and chairs. You could look for a decorative detail that has more subtle style. As for placement, it might go on the short wall where you have a bench or even on the wall where you have your table with two chairs, one on each side to balance the look.

This kind of alteration really requires an architect and a good contractor. The architect will know all the local requirements and how to meet code on everything. You will want/need to insulate the ceilings of the downstairs for sound/fire control, separate the electrical and plumbing lines, and all the things the average homeowner doesn't know or hasn't thought of. In the long run, an architect will save you money. Your friends who have remodeled may refer you to their architects. It's worth interviewing two or three in your home to see which one can help you achieve the best result.

Hi MJ, As per your requirements, you should hire an interior designer. This need to be planned properly. Please let me know where are you located? You can refer the image below to get an idea. Regards Prity