Well, SCSI is the more professional solution, and supports harddrives, optical and magnet-optical devices, as far as that goes, and you can have lots of them. And SCSI is still professionally supported by companies who deal with older servers. But, the devices supported are unattractive today.
DMA isn't really that big an advantage. It caps out at 2.3MB/s and there's no way to go beyond that. And even on 68000 it doesn't really help it boot much faster, you get fast stuttering loads instead that semi-freezes the startup-sequence, since it chokes the DMA to leave little time for the CPU. Today I would say modern IDE is the better solution. I would not have said that for slow MFM/ATA back in the day, it was crap on a stick.
If I wasn't so happy with my CF speeds on (switchable) accelerators, i.e. if I were unhappy using my CF card as main harddisk for all my Amigas, I'd love to try some SSDs on updated IDE drivers. But that would add an extra step, i.e. a card to transfer stuff and then an SSD locked inside each Amiga's case to copy it to. Hurgh. Expensive and stupid.
As for motherboards fitting original cases, yes, they will. I mean, if he makes them tower-only, he will sell basically none of them
and I think the A500 mobo specifically will be much smaller than the original. The other products will very likely be "base model, buy upgrades with it or later" and very attractive, I think. He might carry that approach to the Amiga Reloaded to make them very different from the C64 Reloaded prices, and offer bundles, like complete, tested, new, working, accelerated systems to just plug in. My crystal ball is hazy, but that's my prediction. Cos that would be awesome
The only component left really would be the floppy drive. And now there are USB-enabled, new, floppy drives.