For a one-lens solution, I prefer the 17-55mm f2.8 IS. Sure, it's $900 and the 50mm is $300. But when you compare it with the cost of a 17mm, 24mm, 35mm, and 50mm prime... it's cheap. The Tamron version is decent, too, but bad bokeh and soft corners (not a problem for video). I've heard the Sigma is also very good. Both of these are like $500. Then get a fader ND ($20 on eBay for 77mm). That's the cheapest kit I would consider for a really good, comprehensive set up. Fwiw, I also love the 11-16mm for music video style shoots.
The 50mm is a fine lens (although the 50mm f1.8 is no worse optically, just worse bokeh and a very bad focus ring) and very popular. But it's a bit long. You'll want a wider lens if you shoot in small spaces. I prefer 28mm or 30mm and a lot of major filmmakers (Bay, Woo, Fincher, Kubrick, Spielberg, Jackson, etc.) love shooting wide. 50mm is kind of "boring." But it's a good lens for the money, yes, and you can get nice shallow focus with it, too. Just try out your current lens (the kit zoom, I'm assuming) and see if you can live with it being set to 50mm all the time. If you can, go for it! It's a nice lens.
You'll want an ND filter (58mm I think?) if you want to shoot outside. The ND faders are cheap, but the Hoya HMC NDs are the best thing going for the money (get a couple: ND6, ND1.2).