Kick start voicemail advices


I’m currently running a small Ericsson PBX (around 100 ext.)
I’m targeting a slow transition to IP.
(slow, mostly for the finances of my boss…)

The first step would be to implement a voicemail on an Asterisk Server.
Could any of you point me in the right direction, while I’m making up my knowledge through books and web, so I can start implementing and testing.

What type of card/module do I need between my PBX and the Asterisk ?
For the voicemail feature only, how must I scale my server ?