asterisk users - Apr 2004 - Building a PBX on spain, europe

Hi there, We have to change our PBX to a new one and was thinking about
building it with asterisk.

The things I need it to do are:
  Use a few basic ISDN (European) lines for voice,
  Have a few clients running ip phones (sip, h323.)
  Make it deliver calls to apropiate extension depending on which phone
number external calls dialled into or which line the call come from.
  Have voicemail depending on which extension could not respond.
  Have about 20 analogical/digital extensions.

As far I have tried the second and fourth option a little but know anything
about the others. Is it possible? What hardware, software do I need to
accomplish that??

Thanks a lot !

PS: Also, could you recommend me some ip phones that work well with asterisk
and phones for the analogical/digital extensions??