You can't do it for $400. The PSU is $100, leaving you with $300 for the Mini. If you get a Mini, you need the Intel Mini because you'll need to run Windows for the Nav solution. I haven't looked lately, but if you find Intel Mini's for $300 or less, let me know.
The pro of having a Mini is you can run Windows if you want, you can run Linux if you want, you can run OSX if you want and the hardware package is excellent.
However, you can run all three on your current Windows machine. So, the choice of running a Mini comes down to whether or not you have a compelling reason to run OSX. The best frontend for OSX is ICE3 which is a nice FE but not as flexible and doesn't have as many features as the Windows front ends.
The main feature you will have to deal with is a very basic one - navigation. The best nav programs run on Windows. You'll need to run Windows on the Mac using Virtual box, Fusion or Parallels. The latter two cost about $80.
If you are a Mac user, the pro will be that your car PC is a Mac. It all works like the Mac, is simple like the Mac, and usually reliable like the OSX you already know.