My wife has an iPad and personally I find that it's a real pain to do things that should be easily do-able. I've gone through 4 or 5 apps (some paid and some free) to try to find an app that will just play videos of various formats off a network drive and haven't found a single one that will play all my videos.
Things like 'network drive' are too complicated for a tablet mindset, at least that is what Apple thinks. If you want to use 'network disks and stuff', that is what a laptop / desktop computer is for.
An app that covers your functional requirements for playing all sorts of videos is 'Air Video' or 'Air Video HD'. It requires a server component installed on a base computer, in my case a Mac, but Windows is supported too. The server transcodes the video stream to h.264 which any iOS device can play. I have found that everything I can play on the Mac can be transcoded.
Interface is quite simple. Set up shareable folders on the server and you are ready to go. Support included for viewing with live conversion or for storing the converted file on the iOS device. Among the best two-something Euros I have ever spent.