Learn Italian: Lei è una materialista, signora.

No, it would remain materialista. Nouns formed with the suffix -ista to describe a person's ideology or profession always end in -a in the singular, regardless of gender. To talk about a man, you only change the article, making it un materialista.

How would we make materialista plural if we were talking to a group of women?

It would become le materialiste. While the singular form is identical for both genders, the plural forms split normally: -iste for a feminine plural, and -isti for a masculine or mixed group.

How do we form the word for the ideology of 'materialism' based on this sentence?

You swap the -ista suffix (which refers to the follower) with the -ismo suffix (which refers to the ideology). This gives us il materialismo. Remember that all nouns ending in -ismo are masculine.

Could we drop the word una and just say Lei è materialista, signora?

Yes, you could. When stating someone's profession, religion, or ideology with the verb essere, Italian often drops the article. Without the article, materialista acts more like an adjective. Including una makes it a noun, placing slightly more emphasis on her concrete identity as 'a materialist'.