1) Is there a decent MAC market?
Supposedly. There's a lot less people using Mac's than there are PC's, but there's also a lot less people developing for the Mac, so fewer games you'll be competing with. All things being equal, it would be easier for your game to get noticed on the mac than it would be to get noticed on the PC.
2) How difficult is it to have the same code that compiles for the pc ..... compile on a mac?
Using blitzmax, it's trivial. As long as you keep a couple of basic things in mind (case-sensitive filenames, little endian/big endian when dealing with binary files, using OpenGL) the same blitzmax code should compile without changes on both Windows and Mac.
(Is MacOS case sensitive?).
Yes, similar to Linux.
But all things considering, the PPC mac market is bound to shrivel up to next-to-nothing over the next couple of years, with the release of the intel-based Mac's. so you will have to make a choice:
1) Get a PPC based mac to service the current install-base of mac users with the knowledge that it will only shrink over time
2) Get an intel-based mac which will have much better future prospects, but only a small current install base.