Maya is a general 3d package, Mudbox isn't. While Mudbox is not needed to create models, you can sculpt/paint some pretty intricate details (everything from mountain ranges to scars and tattoos on some axe wielding barbarian) onto a base model and generate textures using this information for use on the same base model in another package(Maya!).

Also, "Must have plug-ins" depends on who you ask haha , I'm sure there's someone who'll just curl up and die if they no longer had Vray or Realflow.

On the issue of building a workstation. Budget aside, is it a case of configuring a machine from an online seller or buying individual pieces and assembling them yourself?