Best LMS for Storyline 360?

Hi there,

I am currently trialing the articulate products for building an online professional development course. I have so far found the software excellent (using storyline 360) but before I actually purchase I would love some recommendations on LMS options that integrate really well with the platform?

Ideally I would like something;

• relatively cost effective
• that offers a payment gateway 
• that allows for organic user growth (e.g. doesn't cap out at 1000 users)

If anyone can share their experiences or suggestions I would really appreciate it as I am not even that sure where to start!

Thank you

Andrea