2 lites of paint on a engine bay i wouldnt even use that on a colour change in the bay

for the top half of the bay your prob need about 1/4 litre, and i think thats the smallest they sell anyhow, but i would get half a litre just so you got some spare.

if your gunna use spray cans, well, i guess i would get at least 6, maybe more cos the ones you dont use you might be able to take back.

dunno what halfords dimond white is like now but it used to come out like a off cream colour from halfords..
if you get spray cans made up dont get too many as they wont take them back