The A&H mixer has 2 headphone sockets - why not just use 1 of them for now? you can add a headphone output in the future if you find you need it! but it is totally unnecessary right now, so I would skip the listen module, unless the a&h mixer is in one room and you intend to use the modular in another room
it will absolutely be enough to get going with and learn with...
Maths is great - as mentioned previously...
Stages is also a great option - I have both - in a case this size you will probably find yourself wanting both - and probably a matrix mixer and a load of stackcables to provide adequate modulation - as you have pams though and could use this for some heavily synced modulation I would recommend getting Maths and a matrix mixer before adding another modulation source ie Stages
Rings and Beads or Rings and another effects module - I like FX Aid XL will make a great addition in the (near) future - but I would wait amd play with what you have and work out what you are missing - it might not be what you think it is...
a disting (mk4 or ex) would be a great addition as you can audition lots of different types of module - especially useful for filling occasional gaps! but don't try to over learn it if you buy one - set up favourites and concentrate on just a few algorithms
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities