This is how i see it so far:
: Turn your old granny amiga 500 to an Amiga 3000 level machine, for ... "SERIOUS" work!
: Run whdload from sd card on amiga 500 as cheap as possible!
If you notice the specs of aca 500 is EXACTLY that! They give you the minimum you will need, for whdload usage on amiga 500!
As for the ability to use amiga 1200 accelerator cards on amiga 500 using aca 500 as a "bridge" for that, imho is a little too far streched! Why give 100's of euros to beef up an amiga 500, when ,with the same kind of money you can have an AGA 1200 based system, with accelerator?
On the other hand, zeus68k has MUCH more horse power and memory (to be precise is EXACTLY "4X aca 500": same cpu in 4X clock speed, 4X amount of memory!) ,and for that i believe it will be more expensive than aca 500, and also it will give the amiga 500, a more "professional" profile.