Built in sound system?

How long is a piece of string? It really does depend on your budget. Twenty years ago, I was really impressed with the Linn Sekrit range of in wall/ceiling speakers paired with Naim amplification rather than Linn. I went with traditional freestanding speakers in the end which I still have. The range is still available but has most likely changed. I think Hi Fi hut are a Linn stockist. There are cheaper alternatives (and dearer), a bit of googling is required. If you do decide to go for on wall speakers, the B & W MT60 D setup is pretty good in my opinion. I have this in my family room. B & W also do in wall/ceiling speakers but I have not heard them. Cloney Audio stock B & W and is probably the best Hi Fi store in Ireland.
Regardless what you go for, run as much cable as you can (think of potential future needs) and be aware that power over ethernet devices are becoming more common place. There are also wireless options such as Sonos which gives you a bit of portability/flexibility. I have two Play 1s and for their size, they are pretty good. I also have a Sonos Connect for connecting to my main stereo. Sonos is subject to dropouts and interference but rarely. Other wireless options I have also used include apple tv, extreme and express and google chromecast all with their own pros and cons. You now have the opportunity to wire your living area. I would go for more cabling rather than less. you never know when you may need it.