Can you build one for me?
I’d rather if you built it yourself, hence the instructions..

How much credit does the windstation use?
It typically uses about $25 of data per year on AldiMobile.

Can I use a different mobile provider instead of AldiMobile?
In short yes, however you will need to work out what frequencies / technologies they support and compare against ITEAD supported table, as some mobile providers may need a different modem chip.
ITEAD 3G Table

Additionally you need to will need to update the APN setting in windstation source code as per your mobile provider’s requirements.