My cousin is getting married in the fall and I'd really like to wear some combination of menswear-inspired suiting pieces rather than a dress (button-down shirt, pants, vest or blazer). The problem is that I've never owned anything like this though I've wanted them for years, and I have no idea what is considered 'nice enough'. What determines 'niceness'? Is it the price tag? Are certain cuts or shapes considered more formal?

Thank you!