planning to change the floor cushions to go with the curtains. can't decide on the colour combination, any suggestions will be great.

I'd suggest dark red curtains if you need curtains at all -- the windows are beautiful as they are, and I'm not sure whether some kind of blind might not give you the ability to darken the room when you want to. On the wall, if you are real fans of any movies or TV shows, three big posters would be fun. Otherwise, I would still suggest using three pieces of art there of that size, all equal in size and somehow relating to each other. Three floral works, or three works in the same style, perhaps. I think a large plain textured rug in a light neutral from one of the cushion fabrics would tie the space together well. You might also consider a lovely low coffee table on the rug to lay out your snacks and drinks for serious media nights.

which type of sofa I should buy ? m confused

I think you need to ask yourself some more questions. If you have children - get something that can be cleaned easily. If you have pets that get on the couch - again, get something that can be cleaned easily. Do you want a sofa that can convert into a bed? How humid is your climate - think of the upholstery and possible mold/mildew if the sofa is not in a location where there is good airflow. If you live in a dusty climate, find one that can be wiped or vacuumed easily. MY first consideration is if the couch is comfortable to sit on!