Word has options to make text Bold, Italic, Size, ..etc. my question is should I only install regular font of a typeface?!

Install all variants.
Word will otherwise distort the font to achieve the effect.

A friend of mine who is a real font guru told me that when Word formats fonts as bold etc. it isn't really that way and is sort of cheating. He said you should always make sure that you purchase all the font variants you plan to use.

For how fonts actually work in Word see https://brochet.co.uk/how-fonts-work...rosoft-office/

Thanks for your inputs, I really apperciate it

thank you!