Welcome to the boards
MissionComputer is what I use for plugins, and what many MacOSX developers use as well.
For graphics, there are a lot of options. I use blender, and get good results with it. It's relatively easy to learn; I learned how to model basics with it in a matter of days and have been improving since. Since there are several blender users around here (check out the Just Graphics forum as well) you would have plenty of good information support for using blender. In addition, it's freeware. Other freeware options include Wings3d and Bryce; I like the Wings3d modeler, but haven't used it for a while, and have seen some great results with Bryce, although I have difficulty using it myself since it is so much different from blender.
If you have a great deal of funds, a great deal of other possibilities open up, including Lightwave, Maya, 3ds Max, Cinema 4D, and others, all of which give excellent results. A good 3d artist, though, can make Blender or Bryce equally as effective as one of those more expensive programs. The most important thing is the artist.
I suggest using one of the freeware graphics programs to learn with, at least.