Router + managed or unmanaged switch

You have a number of issues

First no consumer router has the ability to limit traffic between the lan so there is no way with the router to limit the traffic between a end device and your NAS.

To limit the traffic to the NAS you will need a managed switch but it depends how advanced of a QoS you want. It is pretty easy to limit a port to certain rate and low end manged switches can do that but when you start wanting to limit by users or machines you need a pretty advanced switch. In any case the first thing it to figure out how much traffic the NAS can really pass so you have a basis to divide the traffic. You would then allocate bandwidth to ip based on that number. Even advanced switches are stupid they think if the port is 1g the device on the end can really pass 1g.

The internet is technically impossible to really limit the traffic with QoS....at least inbound where the problem normally occurs. The key issue is that when the path to your house is determined to be full the ISP will decide what to drop by the time your router gets involved what can it possibly do somehow magically recreate the traffic that is important and drop something else.

All the solutions used on a local router are based on the error recovery mechanisms within the tcp stack or the application. The hope is that if the end device gets enough errors it will slow its request rate down.
So lets say I have a 4m connection and I start download a video at 4m and I have a game on another machine that is important running 200k but the ISP drops all the game traffic. To fix this I put in a rule in the router that limits the video to 2m. So what happens to start is I get 4m of video traffic and no game traffic. I then drop 2m of video traffic and give the remaining 2m to the end machine. So at this point I have not solved my problem I am still receiving 4m of video traffic that is eating all my bandwidth. The hope is that the video user detects the loss and reduces the rate it requests the traffic until it no longer see loss. So in theory at least eventually the video machine will only be requesting 2m and it will leave 2m for my game and the ISP will not have to drop it.

This mostly works but it is never that simple. Many applications...torrent for example do not respond and slow down as much. The rate you get to your house is not a dedicated value to start with. Cable systems are shared with all your neighbors so when this connection going to your neighborhood gets exceeded the ISP is dropping traffic from many users. There really is nothing you can do about other people traffic and if you would limit one of your users one of your neighbors could just as easily use up the spare bandwidth as you get it.

The good news I guess is that faster internet is coming for most people. It tends to be impossible to use 50m or larger connections unless you have someone being abusive.....ie running torrents.