I am the proud new owner of a Roku 3 in conjunction with Unotelly.

Over the years I have used DVDFab to convert DVD's to MKV (MPEG). The Roku Media Player App will only play MKV (H.264) or MP4 (H.264).

The dilemma is to:
a) Find a program to convert 630 files to either MKV (H.264) or MP4 (H.264) and then use the Roku Media Player App. I tried Handbrake and I will probably have retired before they are all converted. Yes, you can batch, but I couldn't see how I could queue all the sub-folders in a folder.
b) Install Plex, which I have done. I have installed it on my aging desktop dual core PC, with 2gb ram and on a Mid 2013 MacBook Air with 4gb ram.

The current MKV's, as expected, will play. I have now installed Filebot to try and sort out any inconsistent naming conventions (I have been pretty good because I streamed MKV's over a Playon Mini (hacked to play with Mediator S/W) and used Thumbgen to obtain the artwork).

I have a Seagate 1TB x  1 bay Seagate BlackArmor 110 NAS with the MKV's on it (not all). I also have a 2tb WD drive, with all the MKV's on it (the Media Player App will not see the WD Drive when it is attached to the NAS).
My research says that I cannot have the Plex Media Server on the 110 NAS.

So I have to start up either of the 2 machines when wanting to use the Roku 3 with Plex app.

I thought that I could leave the desktop running 24x7. It is running Windows 8.1 but I cannot find out (i.e. the right setting) how to save power when it is not being used.
I could buy a new NAS which is supported by PLEX but it seems to be a minefield and I don't want to buy a NAS that cannot (successfully) run Plex. Anything decent seems to be $700-$1000 and then there are the disks to buy.

I am now running get_iplayer to get BBC programs.

My Panasonic G10 Plasma TV is not a smart TV.
I have a working MSI Wind U100 netbook (running XP and Linux Mint) , but Plex say that any machine should be dual core.

Any suggestions?

PS. All the above has been gleamed from snippets of google searching and I am no expert in media streaming.
The Roku 3 is wired via TP-Link powerline plugs to the router (also I have a spare 4 x port g/n router).
Ideally I want a centralised solution because I still have the Playon Mini attached to a 32" Panasonic TV in the bedroom via powerline.