Usually .servers are sold bare bone, or with minimal specs. Meaning, typically you choose the CPU(s), RAM, RAID controller, power supplies, NIC options, drive cage type (hot swap or not), HDDs.
Hard to tell from that server reseller you linked above.
You want to think of what OS to have, and if you want to setup a domain (active directory) or not. Like..with server essentials...it's made for a domain. So because of that, it is alrealdy running a little bit "heavy". I don't like having one big drive spindle/volume for a server. I like having a drive/spindle for the OS, and another for the data shares. And I don't mean a pair of drives...in a 1TB RAID 1...and then partition that 1TB RAID into two partitions...because that is the same spindle. 4x drives. Pair of smaller ones up front RAID 1. Pair of larger ones on the back end RAID 1...for the data.
For "just MAS90"...yeah one would think a big fancy server is not needed. But once you have something like Server Essentials....you have:
*Active Directory...a windows domain, and all that runs with that..DNS, DHCP, managing the computers and users, group policies
*File and Print sharing
*User Folder Redirection....users Documents and Desktop folders are redirected to the server. Quite a load here.
*WSUS managing clients
*RWW portal (remote portal)
Usually, a client will grow. Spec them a server that is barely able to run that you think they just need now....and in a short amount of time...it's been outgrown, filled up, runs too slow because it's running too much, etc.
I'd still pitch SAS drives as an option, with some education about expectations of performance. I'm not a fan of SATA drives in servers...they're for desktops. Quite bluntly...7,200 rpm on servers sucks...unless you're running something really light in workgroup mode. Save a couple of hundred bucks up front to spec a server with SATA drives...and then pay some tech several extra hours every couple of months of overtime rates to cover the greatly increased time it takes to manage the server, run updates, reboots, install stuff, etc. And then have unhappy, complaining staff about how sssslllllooooowwwwwwwwwww the network is. I really can't see the savings.
Versus...spend a little more up front for a good server grade disk system....spend less on support over the next 5 years, and have happier staff.
Are all the workstations "pro" versions of Windows?
Also, for disaster recovery, "OEM" WIndows licenses aren't fond of that. Know how Windows wants to reactivate if you clone it to new hardware? Well, image based D/R for servers is the same way. We always do volume licensing for servers OS's. I don't think it'a an option for server Foundation. But with Essentials it sure is. I recall hearing that Microsoft loosened up activation of OEM servers with Server 2012....I'm not sure of the details but I don't want to be spending half a day trying a dozen different Microsoft techs during a clients 911 with me trying to restore a server.
Yes you have options for remote access. RDP...servers have supported that since 2000. And remote web portals such as Server Essentials has. In addition, if you're supporting businesses, having an RMM is a good thing.