As Turkish belongs to Oeral-Altaic language branch It is quite unique when compared to Indo-European languages.I like that my learners believe that it is an exotic language to acquire making it stand out among other languages which have different sentence structures linguistically.
I started teaching Turkish in 1998.
In my experience ,the most common reasons are having a Turkish partner ,for vacation in Turkey and some people have affinity for Turkish culture and people.Majority of my students take Turkish lessons for the reasons mentioned above.
Difficulty of Turkish depends on your native language.If you are a speaker of an Indo-European language ,then you might find Turkish grammar difficult although Turkish pronunciation is pretty easy.On the other hand,if your native language is Hungarian,Finnish,Korean or Japanese you might find it easy as they originate from the same language family.
My students are usually interested in the fact that Turkey reflects both West and East culturally as well as geographically situated between Asia and Europe.They are amazed by the hospitality of Turkish people :)
Nothing that I can think of :)
An enthusiastic,motivated student who can devote his/her time to acquire this language.
Possibly the grammar in which sentence structure is formed differently than most of the languages.
As it is true for other languages as well,they should practice what they have gained as a theory.They should read Turkish books,magazines ,podcasts depending on their levels,watch Turkish channels,speak with Turkish natives .
Although it depends on the learners' ability,eagerness and devoted time and most importantly the mother tongue of the learner ,generally it takes less than 1 year for a devoted learner.If the learner' s native language is French,English,Spanish,Dutch or German it might take a year or a bit more to become fully fluent.