Well, realistically, it would probably take a year to get a ship built in Tortuga at the time. That would be too much realism. Forget realism ... talk balance.
Even ignoring realism, the time penalty on getting a custom ship built needs to be large enough to be a serious inconvenience. The benefits of getting a better ship need to be balanced with some appropriate penalty (besides just cost). Having to shell out a lot of money, and then not getting anything back for a month, would sort of relay the message that this was a major deal.
That would contrast it from, for example, buying whatever battered sloop somebody just sold (after they captured it). That could also get expensive, but you could take it out just as soon as you could get provisions aboard.
On that note ... most pirate-themed games tend to screw up the cost difference between personal items and major hardware like a ship. Let's be sure not to do that. Sure, a fine sword could add up to a year's wages ... but an 800 ton ship, plus 48 brass cannon, well... you get the idea. That's a mistake that really detracts from the feel of the game, realism or no.