First, I'd like to refer to obloquy's Ealar essay
here, since it spares me the task of looking at the sources he used again.
"Pengolodh also cites the opinion that if a 'spirit' (that is, one of those not embodied by creation) uses a hröa for the furtherance of its personal purposes, or (still more) for the enjoyment of bodily faculties, it finds it increasingly difficult to operate without the hröa. The things that are most binding are those that in the Incarnate have to do with the life of the hröa itself,
its sustenance and its propagation. Thus eating and drinking are binding, but not the delight in beauty of sound or form.
Most binding is begetting or conceiving." (O-K, my emphasis)
Obloquy draws the sound conclusion that "Incarnation seems to always be involuntary, except in the case of the Istari, and perhaps Melian. The Istari were Maiar who were incarnated by the Valar as part of their mission, which was to rouse Middle-earth in defiance of Sauron. Melian’s incarnation was probably due primarily to her wedding Thingol and subsequently conceiving an incarnate child. After Thingol’s death, she abandoned her physical body and returned to Aman." (Ealar and incarnation)
Huan, stated to be maiarin in MT, and not certainly rebuked as being so in the later note, would most likely have had a rather high degree of incarnation, since he was also affected by the ruling of limited use of speech (for whatever reason exactly).
What the above O-K quote does not say is whether becoming incarnated by one use of the body would incorporate the other bodily needs as a consequence, even if the being did not use the hroa in that way. We cannot say whether an eala who never ate or drank would need to do so when he/she begot a child. At any rate, it seems highly likely if the form the being incarnates into was one which normally had such needs. Incarnating as a Child of Eru (such as Melian did on a biological level at least, or she would not have been able to beget Thingol's child) would presumably bring the bodily needs of the respective race with it.
As for the istari at least, we know as per UT, Istari essay:
"but clad in bodies of as of Men, real and not feigned, but subject to the fears and pains and weariness of earth, able to hunger and thirst and be slain;"
[ July 23, 2003: Message edited by: Sharkû ]