Is it rude to ask all of this?

No. A professional expects to be assessed, and the ones who find it insulting are telling you something useful for free.

What if they are much cheaper than everyone else?

Ask question 1 and question 5. Cheap quotes are usually cheap because of scope, not generosity — something has been left out, and these two questions usually find it.

Should I ask for references?

Yes, and ask the reference one question: what went wrong, and how was it handled? Every project has a problem. How it was handled is the only part that predicts anything.

How many developers should I ask?

Three. Fewer and you have no comparison; more and you are running a procurement process for a small website.

What if I do not understand their portfolio?

You do not need to. Open the sites on your phone. Are they fast? Do they work? Does the Arabic look right? That is most of what a portfolio can honestly tell a non-technical buyer.

9 Questions That Tell You Everything About a Web Developer

Nine questions to ask a web developer before you commit — with what a good answer sounds like, and what a bad one sounds like. Written for buyers with no technical background.