It seems like an 040/060 board would be the best, easiest option. probably better to go with a new design however, thus getting modern RAM and IDE. new 68060s can still be had from Freescale in various packages and speeds, or you could sell the boards sans CPU. I've got the 060 already lying around here as a spare, waiting for a home :-)
I don't see why this is so unfeasible, I mean, the FPGA hackers around here can make a whole Amiga chipset on one, why couldn't the firmware be made to do an accelerator for those that want it? I'd love to pimp out my 3000 with an 060, heck, I'd even pay to do it for my 500. 040 accelerators were made back in the day that plugged into a 500, and those ran way hotter than an 060.
Come up with a working prototype and the required convincing Youtube videos, and I'm sure you'd have 300 preorders with a quickness.
Get the working proto, BOM and production schedule set up though, before the video comes out. Then you won't run the risk of burning anyone ala' AntiGravity and others.
FIRST product, THEN announcement ;-) Go ahead, buck the trend in Amiga land!
Good luck, my money's waiting!