In the worst case, a smart TV ends up being an overblown monitor. You'd be better off choosing it on the display hardware and image quality and then plugging in a NUC or similar running Kodi, in my opinion.
If you just want access to your local media plus (broadly) web-based streaming, OpenElec will do perfectly. That will pick up your NAS media over an NFS or SMB share or even off the minidlna feed if you like (I'm not a great fan of the latter setup - UPnP doesn't provide the same 'rich media' experience of a fully-scraped, metadata-laden Kodi experience). It will also give you access via plugins to YouTube, iPlayer, TuneInRadio,Spotify and similar.
If you want a bit more, e.g. a browser, then Kodibuntu or a minimal Ubuntu installation with Kodi will give you that. You can also install Pipelight here for some Silverlight-based streaming services.
If you want Netflix, Amazon Prime Video and the works, though, then a well-spec'ed Android box might work, but a Windows platform with SSD is hard to beat. That can boot straight into Kodi for all your main needs, but then launch the DRM-specific applications or a Windows browser for the IPTV/over-the-top services.
At that point, I struggle to see what a Smart TV is actually for, if I'm honest. I have a Samsung on which I use precisely none of the built-in applications, and all it does is nag me about some random service that I never heard of or used being retired imminently. My Panasonic was better, but even then the interface was dismal. Playing music off my NAS over DLNA/UPnP is dire - I'm scrolling for a week to get through half the artists, for example, versus scrolling through a list by artist/year/album/genre, and that's before you leap to something like Yatse on a 'phone to do the controlling for you.
If there's a killer application you know you'll use, a Smart TV may be worthwhile; the dual-tuner DVB-S/DVB-T systems are useful for day-to-day watching; for anything else, I tend to rely on external computing power. It also has the benefit that I can upgrade as technology changes, plus I'm not limited to what the TV manufacturer thought was best for me (e.g. if I want to use H.264 in an mkv, I can, without the TV complaining that H.264 is only allowed in an MP4 container).
YMMV, of course. It ultimately depends on what you want.