How to get from Osaka to Koya-San?

Hello everyone!
My name is Galina and I am a German student finally travelling Japan from next Friday. I have already planned almost everything, but right now I struggle a lot with one of my destinations: Koya-San

I am very confused on how to get there from Osaka... Especially right now with the current railway situation there... I know I have to switch to a shuttle bus on the route, but I could not find a timetable for the train+shuttle bus and also I don't know how to buy the ticket. Since I already got some problems on another part of my itinerary because I was a little too late and the bus was already booked, I decided to post my question, so that everything will work out fine, haha.

My dates:
From Osaka to Koya San
31st March (Saturday), have to arrive in Koyasan between 3PM and 6PM

My questions:
Where can I find a timetable?
How should I purchase the ticket? (I only arrive in Osaka the late evening before, so should I buy it on the morning of travel or will it be too late? Or can I buy the tickets online or from some other japanese city?
)

Thank you so much for any help, I am really struggling on this one.

Greetings,

Galina

This is, use Google translate.
http://www.nankai.co.jp/traffic/info/transfer.html

Koya line is one of Nankai railway's, so you can ask to them.
http://www.howto-osaka.com/en/contact/