the pix has a limited signature set. i believe it is somewhere around 53, where as, the ids appliance is something like 800+, with the ability to write custom sigs.
there is no way to update these signatures on the pix, they are embedded in the pix code itself. as far as viewing the alerts, these are basically syslogs.
your second question is a subjective question. is this a suitable solution. your pix should protect your network pretty well, this really depends on what ports you are allowing inbound.
if you want to try an opensource ids solution, take a look at snort. (www.snort.org). as for a cisco appliance, a lower end 4215 would probably be suitable for a small business, and it shouldn't break your IT bank.
regards,
chris
