Since the game year is 1184, you might not have the time to build a crusade and if any of the crusader provinces are held by people that the Pope thinks are good catholics, he won't let you crusade against them. Just launch a regular seabourne invasion of the Holy Land.
Your biggest problem is going to be holding both the provinces and the garrisons until after 1205 to secure the GA points. You will have a massive counter attack in the 1-2 years after you invade.
Regarding ship building, most ships that require a level 2 shipbuilder can't be built until the High Era and the ships that require a level 3 or 4 shipbuilder cannot be built until the late era, infact the advanced shipbuilders can't be built until the discovery of gunpowder in 1260.
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