If you buy a part of a set, will you get an equivalent discount on the rest of the set?

Yes, but bear in mind that the discount is only applied to the items as a set. So for example, in a 1080AC outfit, the individual pieces cost 400AC. If you buy one of those pieces, buying the rest as a set will cost you 720AC (400 + 400 - 10%), but overall you will have paid 1120 for the entire outfit. So if you think you might want the rest of the pieces later, it's cheapest to just buy the whole outfit at once.