You can use from 200g to 400g, I use 300g, it's quite resistant. To get an idea, usually two layers of 300g gives about 1mm thick.
Tip: The smoother the paper, the more likely it will break during the folding phase. Textured paper is often a guarantee of better quality.
For my part I use color paper brand perfect ideaz. It's a good brand for excellent value for money. Craft Uk is also a good brand that I advise, do not hesitate to tell me your good or bad finds!