We have found it more energy efficient to run one big computer rather than a handful of little workstations and servers - not to mention the cost savings of less equipment to maintain, have depreciate and upgrade. Whether or not that applies to anyone else really depends on how many systems and...
+1 to this!
How valuable is the data? If it's really important that it not be corrupted at all look at ZFS, whereas if it's TV shows and the like you may not feel that level of safety necessary. If you want ZFS there's a couple of easy options - Napp-it is probably the easiest I've tried.
Are you backing up basic files (e.g. user directories etc.) or backing up system images? We just use rsync for user directories and a ZFS on Linux box for those services, and it's pretty darn straight-forward.
I haven't ever heard of that - TRIM isn't always passed through depending on your RAID setup but that affects performance more than longevity, iirc. I don't see why that would be the case.
We have a large number of both in RAID and the SSD fail rate is lower than the HDD rate.
I've used OpenIndiana+ZFS, freeBSD+ZFS and Ubuntu/Debian+ZFS and in all cases the performance of a 6-disk raidz2 could saturate gigabit 2-3 times over which is more than enough for what we need. As such we use ZFS on Linux for reasons similar to yours (better package management and generally...
Sure, as far as I'm aware there's not anything that's significantly better available in that price range. They perform really well, don't use much power and aren't super expensive. I always liked three SAS2008 cards in a 4224 to keep things simple.
As spankit has said, ZFS manages RAM extremely well with regard to using it as an adative read cache (ARC); adding a RAMdisk is adding an unnecessary layer. Using a RAMdisk as L2ARC will also reduce the amount of ARC available as L2ARC eats up a small amount of RAM which would otherwise be used...
You could try a live Ubuntu CD and a program like gddrescue - we've had some succes with that before, imaging broken drives. It has the neat feature of going back and re-trying any bits that didn't work the first/second/third/nth time around so if it's coming and going it may help.