Hiring a SEO agency without getting scammed

I have had interviews with over 15 agencies and I am still confused especially with how different their backlinks strategies and over ideas they say are true. I am gonna list some things they told me and I would truly appreciate if you could tell me if they are true.

1) Correct URLs are very important
2) The best backlink strategy is to build 100+ backlinks a month with 0-10 and 10-20 DA backlinks in the beginning to build a foundation
3) The best backlink strategy is to build 5-15 backlinks a month with high DA authority (30+)
4) Starting a blog is extremely important to rank better. (I run a e-commerce website)
5) I should never buy backlinks

Thank you for your time and effort. I appreciate the help.