Please let me know your tips when you purchase the domain.

How many more DRs?
How much RD against DR?
I'm not sure which one is a good domain.

Check this guide: https://www.blackhatworld.com/seo/what-is-your-checklist-for-domains-for-pbn.1342695/#post-14514468

A good domain usually has a high Domain Rating (DR) and a solid number of Referring Domains (RD) with quality backlinks. Ideally, you want a balance—high DR with relevant, authoritative RDs rather than just a high quantity. If you’re unsure, tools like Ahrefs or Moz can help analyze the backlink quality!

DR and RDs are only good enough for the initial cursory inspection. Check on archive.org to see if the domain has already been repurposed in the past for PBN use or gambling/casino promotion, and check the anchors of the domain's backlinks. Relying on just DR and RDs without any other factors is pretty much the same as buying blind.