You want a dual-band concurrent (a.k.a. "simultaneous dual-band") model that can still support the cluttered 2.4GHz band for your legacy devices, but also supports the cleaner 5GHz band for modern clients.
Also, since it's 2014 and not 2007, you should look for 3-spatial-stream (3x3:3) 802.11ac, with support for 80MHz-wide channels, for the 1300Mbps PHY rate. A lot of good 3x3:3 802.11ac AP products came on the market in 2013 and here in early 2014 for less than $180.
The problem with trying to pay just $40 for bargain basement crap like the MR3420 is not only that you get slow 7-year-old Wi-Fi technology, but you tend to get products that skimp on quality in order to hit a price point. For example, a 3x3:3 802.11ac AP would have high-power power amplifiers (PAs) on transmit, and high-quality low-noise amplifiers (LNAs) on receive, for maximum range. But a bargain device probably wouldn't have those features, and range would suffer as a result. I'd rather pay a little more to fully modernize from time to time, and get something that was the flagship product when it came out, so you know they spent some effort making sure it works and performs great. To me that's a lot better than staying constantly on the losing end of the technology curve just for the sake of low up-front cost.
I second what another Answer said: Considering using a streaming protocol to stream video, rather than a remote filesystem protocol. Remote filesystem protocols like SMB have a lot of overhead so that the remote share is capable of everything a local disk is capable of, like random access to files, access to file permissions and ACLs, and write access. These aren't things you need when streaming video, and they cause overhead that gets in the way of efficient streaming. Instead, use a streaming protocol like DLNA or iTunes Home Sharing.