I took a look at some of the T1700 offerings. The following is based on -> this listing. Here's the spec sheet.
To check CPU bench marks, look here for Passmark CPU scores.
The Xeon E3-1225v3 3.20GHz scores 7229
That's more than enough to run OMV and Plex, even with a transcoded stream to a portable device. You could easily add-on Dockers and, with 16GB of RAM, a VM or two.
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The design of this box appears to be as a workstation or a SOHO server, similar to a Lenovo TS140.
Of the Dell T1700's I looked at, none specified ECC RAM. ECC is not an absolute requirement, but it's a good thing to have.
While the T1700 has 4 SATA ports on the Mobo, it can only accommodate (physically) 2 each 3.5" drives. (Here's one with an interior view.) Note that, in the consumer market, high capacity 2.5" drives are still a bit more expensive than 3.5" drives of the same size. Still, if you boot from a USB stick, 4 ports (with 2 each 3.5" and 2 each 2.5") might be enough for what you may want to do.
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As always, on Ebay and anywhere else in the used market, "Buyer Beware" always applies.
What you may lack in knowledge, of the various used platforms, can be made up for with lots of research. (The above is a brief example.)
Generally speaking:
Xeon's are good - they support ECC
ECC is good - it keeps memory processes and files cached in RAM clean.
Space for 4 or more physical drives. Four should be enough for most NAS use cases.
Watch out for older RAID cards, if supplied. Some won't do JBOD and some won't pass SMART stat's transparently.
(The upside here is, 8 port used replacements can be had at bargain basement prices.)
Really old Xeon's may lose kernel support down the road, but I'm not worried about it.
BTW, the E3 series (starting in 2014) is a generation or two newer than what I have, with X5660's (which were offered starting in 2011) . -> Xeon's
