answering your numbered questions and then the baseline one ! !
1. i started at 12-13, and i believe it took me two and a half to three years !
2. started on just one in those single juz, large font type of Quran, gradually increased to 3 pages of standard 30-juz Quran with smaller print .
3. revision ! memorizing is easy, the revising and revisiting part is a whole different story . i didn't actually overcome it . . . it's really important to always revisit, revise and touch up regularly !
4. surah Kahf ! there's many reasons i'm drawn to it, really ! it's a constant surah in my life because i read it every friday, and i love listening to its tilawat (recitation) too . the four different points drawn up in the surah are very interesting, and the meaning of the surah in it's entirety really sticks out to me . part of my favouritism to it can't be explained . . . i'm just drawn to it, and very strongly ! aside from that, surah mulk is one of my favourites because it was my favourite to memorize in the 29th juz
!
back to your original question ! to go on an 3limah course means to study Islamic jurisprudence, right ? and be knowledgable on various Islamic topics and essentially become an Islamic scholar ? if that meaning serves right, then i'd definitely say to do hifdh first . these Islamic laws and beliefs will be drawn out largely from the Quran itself ! think of it like how highschool progresses : you generally don't start strictly studying for a GCSE until year 9 ! in years 7 and 8, you're developing baseline understandings of topics you'll later expand and build on in year 9 ! i believe it's the same here, too : having a strong background in the Quran will really help you in your 3limah course ! the course couldn't even exist without the words of the Quran to give it a foundation .
but with saying that, there's a difference between memorizing the Quran vs. understanding it ! there is no way anyone can discredit the rewards of memorizing the Quran, but to understand it and implement it is equally as important; especially considering you want to do an 3limah course ! so do keep that in mind when making your ultimate decision, too !
tldr : first do hifdh and understand what the Quran is trying to teach you, then go on to do the 3limah course that will branch out from what you've understood from the Quran !