You absolutely must have a graphics card to run Mac emulation on an Amiga, otherwise the emulator has to run the usual chunky-to-planar conversion, you're stuck on a two-colour screen, and it is much too slow to be usable. If you have the Mac ROM, you'll get a "?" symbol on the screen, but the emulator won't do anything until you've got Mac OS installed. System 7.5.0 (or .3, not sure which) is freely available from Apple's website, but you have to dig around in order to find it.
I've got Shapeshifter running on my A500 (33Mhz 020, no graphics card), and it's pretty fast in two-colour mode. I was playing a 3D driving game called "Vette" and it looked like it was going nearly full speed. Clarisworks and some other applications were also very usable. It's only when you pick a screenmode with more than 2 colours that it gets butt-numbingly slow.