I would imagine the 68000 would be a LOT easier to get than a 68030. Also, once you start getting into the realm of fancy 32 bit parts like the 020/030/040/060, it starts getting a bit more complex in terms of the stuff you need to make it work (heck, most of the 32-bit parts have an MMU and can run a full-on Linux Kernel)
And I wasn't thinking OPL for the sound, I was thinking of a sound system more like that used in many arcade games that used a YM2151 FM synthesis chip and another chip to handle raw sound samples (used in the arcade games for sound effects and percussive samples that the 2151 couldn't do)
Since storage space isn't an issue (due to the inclusion of a hard disk and floppy disk) there is no need for the sort of sound chip these arcade boards used (usually something like an OKIM6295 ADPCM chip) and a cheaper simple DAC could be used I guess.
And assuming the rest of the system is built properly (and the 68k is fast enough) the tight timing many of these arcade boards had that meant the sound hardware had to be controlled via a separate CPU such as a Z80 isn't an issue.
Biggest downside would be the lack of music written for the 2151. (what music exists for it most likely exists in the form of the ROM files for the arcade games that used the 2151) That and the fact that you would probably need a YM3012 Stereo DAC chip or similar to do anything useful with the output of the 2151.
EDIT:
I should add that you probably want the 68HC000 CMOS version rather than the 68000 HMOS version as the HMOS version was discontinued in 1996 or so but the CMOS version may still be in production (or if not, certainly easier to track down)