There will be at least one ship that is the best you can acquire through pure use of cash. I predict that the best ship that you can get period will not be bought but rather found/earned. That's just kind of how it works in this genre: cash is easy to acquire in large quantities after a while, so the measure of having earned something powerful is usually to be able to clear some sort of quest to unlock it rather than throwing a pile of money at it. That's not to say the ultimate buyable ships won't be damn good ships, but if you can't settle for anything less than perfection you probably won't even find it at my shipyard, which is designed around offering ships that come with hefty price tags in exchange for well-rounded performance.