I have a slightly less powerful machine (Core2 Duo E4700 @2.6ghz, 2 GB RAM) that can do everything you have listed. You should be fine.
For plex it won't quite be able to transcode 1080p, but doesn't sound like you need it. Just set plex media player on any connecting PCs to allow direct play and direct stream. This way the front-end machine will do the heavy lifting and put very little load on the OMV server. It will be able to transcode for 1 or 2 tablets just fine.
Don't bother worrying about the video card. You will use it once to install and then do everything from a web browser. My setup doesn't even have a video card...