I would like to do this - initial questions

Looking at the installation instructions, this does not seem too difficult at all. Before I get to my questions, a brief look at my proposed setup:

I want to interconnect 4 offices with the system (25-30 phones).
I propose to host my asterisk server at my ISP, for speed and reliability purposes. I do not want to use analogue backup lines here. All outgoing calls (6 VOIP lines) will happen here.
Each office will have its own analogue line for backup.
Reliability is key.

As I am about to jump into this, a few initial questions strike me:

  1. does it matter what hardware I use (computer-wise)? I realise speed matters, but are there compatability issues with certain motherboards/ram/etc.? Any reccommendations?

  2. same question, but for OS. which version of unix should i install?

  3. are the commercial products out there worth it? Should i skip the whole thing and just buy a trixbox? should i go even further and subscribe to a service like fonality?

To be clear, I am somewhat experienced setting up systems and i don’t think asterisk/freepbx will be a problem. that being said, if an installation and setup of freepbx gives most experienced users problems i’d like to know.