Looking to build a low budget HTPC / NVR that takes a 2.5" Hard Drive - under $200

In recent months I have been doing some security camera installations - lately I have been using the Ubiquiti Unifi Video System. As part of this system, there is an NVR which runs about $300 or around $200 if you know where to look. It features an Intel D2550, 4GB of DDR3, an Ethernet Port, USB 3.0, an SD Card Reader and most importantly a 2.5" laptop hard drive (this makes it easily upgrade-able).

I am now installing some other systems and need to build an NVR for them.... after searching around, I am beginning to think that it might actually be cheaper to reformat a Ubiquiti NVR and use that than to build my own "mini pc" for this task.

The obvious kickers here are that it must be under $200, low power, small form factor and have an easily upgrade-able hard drive - preferably a 2.5". HDMI out is also a bonus if possible.

Has anyone here recently done a build like this? If so, what sort of equipment would you use? So far my mini-ITX builds have come out bigger, more expensive and more power hungry than this little guy has.