Any answer would be highly subjective regarding necessary. I prefer to build a wide array of ratings/sizes of ships so I have some diversity; I run most of the games when we play so I am always thinking about scenario creation. Others prefer building along the lines of time periods or conflicts. Others, yet, prefer frigates or SoLs and the types of battles they engender.

With the starter, you'll have one SoL and one frigate on each side. I don't think you would go far wrong picking up a second of each. This would give you many playing options - four against four, SoL against two frigates, duels, etc.

The Victory and Constitution are nice given their historical importance.

You'll find that once the game starts, you can easily substitute any ship for another by using various ship logs as most people will never know the difference; people will focus on their maneuvers. Given the level of abstraction already present, I would not be too concerned about the "accuracy" of firing arcs, etc., when substituting, though if you have the skill, you can always create customized ship cards.

Basically, start playing and build according to what you want to try next. I am taking the gang through WWII chronologically, and that is providing me a scheme for building my WGS collection.