I've looked around, but I've either not noticed it, or it's buried somewhere in the vast wastes of the internet, so I figured I'd post here and see if I can get an answer.

Do you need the Most Wanted Expansion, to fly the faction's ships? As in, if I were to buy a StarViper, or other ship from said faction, would I be able to field them, without having bought the MWE?

Most Wanted will let you fly Scum Y-Wings, Z-95s, Firesprays, and HWKs. The other ships are all self-contained.

There is nothing in Most Wanted that you are required to have to play Scum & Villainy.

If you buy ships individually, however, you would need enough individual Scum & Villainy ships to field a complete list. S&V are not "mercenaries", they cannot be taken as part of an otherwise Rebel or Imperial squadron and can only be fielded with other S&V pilots. Most Wanted is a very cost-effective way to get those pilots, especially if you have an existing collection.