Here's the thing...

If you don't think you'll need to sculpt into the 10's of millions of polygons then both Mudbox and ZBrush may be a bit of overkill.  Don't get me wrong, I'm not trying to talk you out of them but I am trying to provide alternatives at the lower end of things:

Hexagon, Silo, Blender

These three apps all have displacement painting. With the exception of Blender, the interfaces in them are very easy to jump into. Silo, Hexagon and Blender will allow you to sculpt ino the millions of polys (about 2 million is the max for them I think).  They all fall in the $150 or less range (Blender's Free).  They all do more than just sculpting.

So, that's something to consider to those out there that may not need all the bells and whistles and who are also concerned with price.