"Devoid of a head, or any features at all save for four short, clawed legs, this creature’s body looks like a large, glistening brain."
is the description of an Intellect Devourer.
Adding the proboscis for the new description.
So I would judge that no hands (but Claws if they have proper digits?) no neck, no head or headband, no feet, no eyes, or in the end any part it does not have it cannot wear when it is not possessing a proper body.
That being said I might give it 1 Ring for proboscis. Any slot-less items too are good like say Ioun Stones!
So it might get away with a Shoulder slot, bracer slot (maybe 2 sets?), gloves (Maybe 2 sets?) and maybe a belt. But beyond that it could use the host body's magic item slots instead?