I'm not sre if HS looks after backups for the clients. If it does then that saves configuring them individually. Of course Ubuntu server is a solution I have used if you like a challenge (good user forum).
Cat5a should do you for 1GB/s and you should use it to the server/NAS. For all units then you will need a 1Gb/s capable switch/router. 100Mb/s clients should not be a bottleneck because with careful config. they will not be demanding b/w all at the same time. Your router and exchange line will be the bottleneck I suspect.
I setup my server in the garage and manage it from a browser.
Internet security for the kids is best IMO left to the browser and ISP safe surfing features otherwise you'll always be wondering if you need to tweak it!