Use an old iMac as a TV

I am in the process of evaluating the purchase of a new iMac. At the same time I am trying to re-purpose my old iMac as a TV (I do not need a second monitor) in my bedroom, which is some way from my router. The iMac is a 27”Retina 5K, 27-inch, Late 2014 model. My idea was to use something like a TV USB Tuner which I could plug into a TV aerial output which is already in my bedroom. Roughly speaking I would want to use an RF lead to plug into the UBS TV Tuner and then input the USB TV Tuner to my iMac. The choices seem to be there, but with many pluses and minuses and variable feedback from purchasers. Has anyone tried something similar and, if so, what advice would you give? Thanks