What Gav2k said. Especially Early 2008 and earlier as they have the Intel GMA X3100 or worse, the GMA950. If you are going to go with an '08 at least get the Late 2008 with the 9400m, it will still be able to play HD flash videos at a reasonable frame-rate. I own both, so I should know 🙂
Even then, something with an i5/HD3000+ would be a better bet but now you're looking 2011 and up.
My Early '08 MacBook is locked in on Snow Leopard and Windows 10 (which runs quite well) with an SSD. It's a great backup laptop for office work and browsing around, but not so much for streaming HD content. The Late 2008 is on El Capitan with an SSD + 8GB RAM and it still runs beautifully.
If you just want something on your TV, you'd be better off with an Chromecast/ATV/Fire stick etc. They're more purpose built and much more efficient.