So, having got my hands on the old machine today,, I've been playing around trying to get stuff up and running. Got lost in permission-hell, not 100% sure what the problem was or how I solved it but it seems solved for now. Decided to forego RAIDing entirely, I can just distribute my shared folders among the different drives and maintain that manually, also works to keep this as basic as possible while I learn the ropes, so to speak. Once I get the file/media server bits sorted how I want them then I look at sorting out a proper backup using rsync.
I've been able to get SMB set up and user with read/write access, can drop files on from my laptop, and read them back fine - I'm using a small video file as a test. So OMV <-> Windows is working fine. Now need to figure out how to get my Samsung TV and phone connecting. A bit of googling suggests that - for the TV at least - I need to play around with some DLNA stuff, so a bit more research to do there. This has a been a fun little project so far, despite the banging of my head against the aforementioned permission issue.
I am not doing any real transcoding on the machine. But, you could get a reasonable video card for $40 and off-load those duties.
As @ryecoaaron mentioned, I'm not sure I even need to transcode at the server end. If I did, luckily the machine came with a GTX 760 so could always pop that back it and should be more than capable
If this process has highlighted anything so far, it's tat I should get a bit more comfortable with the Linux console. Had an issue with a HDD with bad sectors, didn't clock it for a while and had to fall back to diagnosing it with my Windows laptop and a HDD dock. I know I could've done the same sort of thing just ssh'ing in to the box and running some stuff.