At the expense of getting brickbats hurled in my direction, might I
suggest one of the Freescale Coldfire development boards (or clones
thereof). The Coldfire is basically a 68000 with some of the
instructions removed and others substituted. The devkits all have
USB and lots of interesting goodies.
You've got up-to-date development software to boot.
Learn to program the Coldfire, and the shift to 68K should be mostly
a matter of dialect. The basic architecture's the same.
Or, just get an old Atari ST--the operating system's easy
(essentially a duplicate of MS-DOS in many ways) and there are good
tools available.
For what it's worth.
--Chuck