And what exactly would that personal folder contain? What types of files? If you're using a "smart" TV with an embedded operating system (Android? Something else?) you'll have to make sure that whatever file formats you're dealing with are supported by the TV's software. Otherwise you're back to square one and looking at attaching PCs to each TV.
What I would recommend for you in this case is going to heavily depend on exactly what kind of use cases you have in store for the nebulous "user" connected to the nebulous "smart TV". If it's just watching videos and viewing pictures, you should be fine. If they're going to want to start custom Windows applications, you immediately either need a remote framebuffer solution (e.g. a thin client setup), or a thick client setup with an actual PC at each workstation.
I think I would need a server OS for that (Windows Server 2012?)
Incorrect. For the number of users and folders you are proposing, you would definitely not need a server operating system. Just create a few users on Windows 7 and share a few folders? Any client OS will work fine for that. The question is, what types of files are you going to use, and what are you going to do with the files once you obtain access to them. This is all just part of the standard Microsoft shared folder infrastructure (sometimes referred to as Samba or Server Message Block (SMB)). So far you haven't given any compelling reason why this won't work just fine on a client OS.
However, I doubt there's a smart TV that could do anything meaningful with files other than videos and pictures once accessing the shared folder. Authentication/permissions are not an issue, it's software support on a "smart TV" for manipulating the files as needed that I'm doubtful of.
You might be able to get by with an Android-based USB stick, like the FXI Cotton Candy or something newer/cheaper/better but similar concept. Basically what you do is plug the HDMI end of the stick into the TV (the TV doesn't even have to be "smart" at all), connect your peripherals to the stick using a USB on the go cable fed into a USB hub (or just use Bluetooth peripherals), connect the stick to the WiFi network, and you have a full fledged operating system (usually based on Android or desktop Linux such as Ubuntu). If the OS doesn't have the software you need to run, you could opt to download a remote desktop client and have the users remote desktop into the server.
Now, one thing I should caution you about. Having more than two simultaneous users connected to a computer via Remote Desktop starts to get VERY expensive, VERY fast. Not only would you need a Windows Server license, which is much more expensive than a client OS, but you would also need to buy a Client Access License (CAL) for each additional simultaneous user. If you do decide that remote desktop / thin clients is a better solution, you might look into running some distribution of Linux on the central server, where you can avoid the licensing fees of Windows.
I am specifically avoiding going into details of the capabilities of specific Smart TV or thin client devices, because this question is so vague that it is borderline NARQ. If we start discussing various devices, this would become a shopping post, which is off topic for SuperUser.
If you're looking for a conversation similar to one you might have with a salesman in a retail store (except with less vested interest in getting you to lay out a lot of cash for something that isn't really what you wanted), stop by SuperUser's chatroom, Root Access.