If one's zakaat is due, however one anticipates a huge expense i.e. buying of a car, should the value of the car to be purchased be subtracted from the amount due before zakaat amount is calculated?

No, this amount should not be deducted. Zakaat should be paid on the entire amount. However, if you purchase any of these items before the end of Zakaat term, then this amount may be deducted. (Ahsanul Fataawa vol.4 pg.301)

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai