What parts do you need to test the mainboard? imho you only would need a power supply, you don't need the daughterboard, mouse, keyboard, floppy or a harddisk to see if the thing works. Pop a 2 or 4MB double-sided simm in the first simm socket (chip ram), hook it up to an Amiga monitor via the 23 pin rgb port and it should give you the install disk boot screen. There are only two easily replaceable chips on most A4000D motherboards, the two rom chips that is. Superbuster may be in a PLCC socket for easy replacement, but use a proper puller tool to remove/replace it. If it doesn't give you the boot screen, and instead all you see is black (bad), or grey (not much better), you've got problems, for all the other chips are surface mounted and not easily replaceable for the average home solder tech.
For a floppy, you may luck out and find as I did that a standard pc high density floppy (Sony, Mitsumi) works as df0: to boot the machine into workbench (though from that point forward you'd probably have to use the diskchange command to switch disks or an external floppy drive on df2:).
Oh, I almost forgot, you need some kind of cpu card installed (A3630, A3640, etc.) and set the jumpers accordingly for the internal/external clocks.
