Learn Italian: L'affidabilità per noi è importantissima; non compriamo mai un'auto qualsiasi. - Reliability is very important to us; we never buy just any car.

Placing qualsiasi (or qualunque) after the noun adds a sense of emphatic indifference or even low quality. While qualsiasi auto simply means "any car," un'auto qualsiasi means "just any ordinary or random car." Notice that when it goes after the noun, you must include the indefinite article (un').

Why does un'auto have an apostrophe if auto ends in an "o"?

Even though it ends in "o", auto is short for automobile, which is a feminine noun. The feminine indefinite article una drops its final "a" and takes an apostrophe before a noun starting with a vowel, creating un'auto.

Could I say molto importante instead of importantissima?

Yes, both mean "very important." However, adding the absolute superlative suffix -issimo is an extremely common way to express a high degree of something in Italian. Remember that it must agree in gender and number with the noun it describes, which is why it ends in -a here to match the feminine noun l'affidabilità.

Why do we use both non and mai in the second clause?