By way of update, I'm going with Plan A above:
Use Logic Express for recording and basic mixing, then do any post-production audio cleanup on the PC with Audition, which has just such excellent tools (e.g. spectral editing, the healing tool). The Mac and the PC will be on the same network so I can use Audition to work on the file in-place on the Mac. Did a quick experiment today using my Macbook Pro and it worked quite smoothly.
It seems the most straightforward, since both tools will be running natively. Ordered iMac, Logic Express and a nice Firewire cable, so by end of week or so I should be up and running!