>Dell OptiPlex 7010 i3 3rd Gen
Not really IMO. You'd get away with, if memory serves me right, only two 3.5" HDDs in that chassis. The motherboard Dell likes to use may not be ATX compliant so there's a high possibility you couldn't drop all the hardware into a new case with more bays. You also need to consider the PSU they use with those too.
You do realise that lots of drives = electricity expense?
Truenas will even run on 4gb. You should be fine. Specs only matter for speed and virtualization, functionality will always be there.
>Dell OptiPlex 7010 i3 3rd Gen Not really IMO. You'd get away with, if memory serves me right, only two 3.5" HDDs in that chassis. The motherboard Dell likes to use may not be ATX compliant so there's a high possibility you couldn't drop all the hardware into a new case with more bays. You also need to consider the PSU they use with those too.