milspec = military specifications
In other words it is not upgradeable or very little can be upgraded. It is cheaper in the long run, but with very little customization. It is entirely possible you did buy one of these, and if you did, then the Jump Drive should already be in it.
Now that being said, if you have a save before you bought the ship, reload it, and head to like, New Detroit. There, go and buy a STOCK galaxy, and upgrade as you see fit. Don't forget to buy a new radar and sector maps.
From what you describe I'd bet you did buy a Milspec version of a galaxy.