I think what you face is the proverbial challenge startups run into of how do you demonstrate value without users and how do you get users until you have value.

Online training is a great niche market with the opportunity for huge profits. At the same time its an increasingly crowded market place so you need to really focus on what is going to differentiate your business (see: http://sellyourcourses.com/is-selling-your-courses-a-good-idea/).

If I were you I would focus on content first. If you are willing to invest in taking the trainers material and building an online course out of it and sell it on you platform there isn't a huge down side for the trainers. That process can take a while and be pretty involved so you are offering to do a good amount of work at no cost. In that instance 50/50 seems fair.

Udemy's model works better if the trainer is on the hook for developing all the content since they are doing the leg work - but honestly if the trainer can do all of that there are a lot of different platforms they have at their disposal to sell their content.

In any case I would start with a few key courses and get a minimum viable product online. Don't start by trying to build a huge library. Get a few courses online and see what happens. This will allow you to validate your idea and you will learn a lot based on the feedback you get.