What are the main questions I need to ask for an e-commerce site in order to send a proposal?
Very hard to answer 
Unfortunately this is the kind of thing that gets much better with practice. I’d look online… I’ll try to think of a few points:
- Approximately how many products
- Do they have few enough products to upload manually or are they going to be importing
- Do they already have them organized in a way that will work for the category structure on the site, or do you need to do that
- Do they want filtering
- What currencies do they need to accept
- Do they already have a payment gateway that will work online
- Do they need to integrate with any third party services such as fulfillment centers/accounting/etc