Since the color is more modern, try to stick to modern furniture. But that color could also be contemporary so you can also use contemporary furniture

Hey! Great space! The table is too small for the space, it would immediately help if you had head chairs on either end of the table. It would read larger. If you're interested in a new dining set, try a round dining table, seating at least 6 for size. You need something to define each space, a new furniture arrangement could help. I would try rotating the sectional so that its off of the window. Good Luck!

Can I make a suggestion: Is is quite hard to gauge the space / layout by the two photo's you have posted. Have you considered adding a picture of the floor plan even if it is just a hand drawn rough drawing.

I think it's always good to have room for a study, but that's because I work from home and like a dedicated space to it. If it's always going to be used as a guest room/study and won't get much use, maybe removing it completely is best.