I know I'm gonna get modded down for this - so be it:
Typical /. radical evangelism for open source, at all costs (metaphorically, not $$), without regard for the whole of the circumstances.
If there was a dedicated IT team, fine. If this was just OP and his grandma only, fine. Any of several circumstances, fine. But that's NOT the case!
Here we have lot of users, you MUST have dedicated support, and OP can't (trust me, I've been in this situation) provide that 24/7 long-term. And keep that server running, but that can be outsourced very cheaply is a delusion. Who's gonna pay for the next X years?? IT Consultants aren't cheap, and any upgrades that break things will be costly to repair, while being an outage for the users.
In a situation like this, COTS, with consumer support available and used to dealing with non-technical users (you know, the helpdesk script monkeys that piss US off...), is the way to go.