dimensioning asterisk (with resilliance?) for commercial use

Hi all,

I have been approached by a company who are moving premesis and are looking to replace their entire legacy telephony
system. Whilst I run asterisk here at home and have built quite a few small systems for several SME's, Asterisk is not my core activity, and I could use some help!

I am certain that asterisk will meet their needs from a feature set perspective, but have some questions regarding dimensioning and reilliance. I also have a major headache sourcing a way to significantly cut the costs of calling uk mobiles from it ...but thats a separate issue!

The company has/requires:

2 x ISDN2 lines with DDI;
1 x POTS line usaed for FAX with ADSL for general net access;
1 x POTS Line with ADSL dedicated for CITRIX Access;
currently around 32 extensions, with a requirement for 48 extensions;
voicemail;
they also want SIP/IAX WIFI phones for the shop floor (LARGE mechanical workshop!)

I'm wondering:

how best to dimension CPU(s) / RAM / HDD for a system of that size, I'm "guestimating" something around 4GHz+ with 2-3gb of RAM and a decent sized system HDD in RAID1 - any particular hardware reccomendations / turnkey boxes ?

If there is any way to factor in redundancy (e.g. HA linux)
to ensure continuity of comms in a system failure scenario.

If anyone has any particular reccomendations for channel banks or other ways of grooming that many extensions back to
the asterisk server.


Many thanks in advance.

Regards

BB