With the 2950 getting to be old hardware, there are quite a few becoming available at very low prices. I have one running on Linux Mint 17.3, with a SATA RAID array, and it's working. There are the usual problems, such as fan noise from so many small fans moving a lot of air
1: You don't need an interposer card. The caddy has two sets of screw holes, one marked for SAS. The SATA set is displaced to give room for the interposer card. Use the SAS position.
2: The interposer card does allow higher-speed operation. It seems pretty obvious that any RAID array should have consistent speed, so using interposer cards would be all or nothing.
3: The drives I am using are rather old (I am trying not to spend money) but are the same model, with the same firmware. And I have spares.
4: There are two SATA connectors on the motherboard, and it would be possible to connect an eSATA backplate, though clearances for the connector look tight if you need to have an expansion card fitted.
5: There are some old SAS drives about, relatively low capacity but sometimes a cost-effective way of getting a drive caddy. Do you want to rely on one? A set of new SATA drives in a RAID array does look a better bet. The safest option may be mirroring, rather than doing anything more complicated with the data. Just how long will the hardware controller last?