The majority of the population were Aboriginal, but they were extremely reluctant to do labor outside of fulfilling the need of their villages.
Therefore both the Dutch and the Spanish at some point paid coastal Han Chinese to move to Taiwan. They especially needed Han laborers to provide food, hunt for deer skin, and mine for gold.
Aside from priests who wanted to spread Christianity to the aboriginals, most of the colonials spent more time dealing with the Han laborers.
The Dutch and the Spanish actually did this first in South East Asia, and only replicated the model here in Taiwan. The 1603 Chinese laborer revolt (Sangley Rebellion) in Manila predated both the Dutch and Spanish occupation of Taiwan. Most of the 20,000 killed during the massacre were Holo speaking Chinese. So the Spanish had a lot of dealings with Holo speaking Chinese laborers before coming to Taiwan.
At the height of Dutch Formosa, census showed that at least 70% of the population were Aboriginals, with less than 30 thousand Han Chinese and about 2000 Dutch.
Prepared, no. Wrote some dictionaries once they got here, yes.
There are several Basay dictionaries written by Spanish fathers, Vocabularino de la lengua de los Indios Tanchui en la Isla Hermosa, Doctrina cristiana en la lengua de los Indios Tanchui en la Isla Hermosa, Arte de la lengua de Formosa, and Vocabulario en la misma lengua.
They are however lost for some reason... If they ever find these books it would be pretty awesome.