But do you really want to put a NAS into a router (with WLAN???) in the (passively ventilated) case? You shouldn't deliberately expose these things to premature heat death. Without active ventilation, only as few active components as possible should go in there.
I only have the switch and the modem in there, and the modem isn't even mine.
6U are enough, as long as the NAS doesn't have to go in, although that would still fit now. But then it gets tight. No more 19" space would be available.
I got the server cabinet from a new goods supplier on eBay for just under 70 euros. It should be easily found with the picture. The patch panel was brought by my electrician, but it's nothing special.