Golly gosh, perhaps an english lesson first! At least you're taking a step in the right direction with Datsuns!
The 120Y and 1200 are very similar mechanically but in terms of bodies they are very different. Basically (in my opinion) the reason that the 120Y will be so much cheaper is that
a) they are more common
b) they are so ugly!
The 120Y is a more unusual choice for modifying, though it's good to be a bit different, a mate in Nelson has a 120Y sedan with a worked A15 and 5 speed which is quite impressive, the main practical downside (again in my opinion) is that it weighs about 100kg more.
I don't imagine that there are any CA18DET powered 1200s with sunny boxes?
I think that the reason that it's a popular swap is the good power and reasonably light weight, along with readily available power upgrades.
Anyway this lot is just my 2 cents worth!