It's really best if you contact them by phone to ask any questions, it's certainly the quickest way. Email should work, but wouldn't be my own personal preference.
Some people like to work from a set template when it comes to asking questions, but it's not required, you just have to ask questions that are important for you. With a wholesaler, you would discuss their trading requirements and see if they suit what you need. For example, is their MOQ suitable for you, is their account system what you are looking for and so on.
If their MOQ is a little too high for you, then discuss possible options where by you may be able to come to an agreement to do business with each other. The most important thing to have is a clear understanding of what you want, then you know exactly what to say when you contact them.