Is your NAS drive shared using samba, FTP or http? For now, to stream movies, only http seems to be supported. With FTP and samba, you need to download the movie first to your iPad to play it.
Since you have your movies in AVI/Mkv format, AirPlay is out of the question so an apple tv would not help. You can get a tv out cable so you could play those AVI/Mkv files over http with your iPad connected to your tv. But buy the real Apple tv out cable to avoid compatibility issues.
You could have a look at Boxee device or any that can connect to some NAS drive and control the device with your iPad.
In my case, I converted all my movies to mp4 format because I have an apple tv 2. So I can use iTunes home share feature on a PC and the apple tv is able to connect to it.
For apps, have a look at OPlayer HD. Free and pretty good...
VicoPad addict!