You can get expired domains and either restore a site from archive.org or create a new one with fresh content. Niche specific is good, but you can manipulate it with content if not exactly your niche. I check the metrics like TF and DA, but also manually check the site history and backlink profile for possible spam to avoid it, which IMO is even more important than the metrics.
Something I recently posted in a PBN footprint thread that should sum up where to start from:
If you have a small PBN and diversify your project's backlink profile with other types of links, then most of the times you don't need to worry too much about footprints. Avoiding PBN footprints is a must, but remember - the more effort you put into it, the more expensive it will be for you (time is worth too!). There is this thing called budget, that is also important in SEO. It's a numbers game, you need some amount of good PBN links that work, are indexed and don't leave footprints for G. In general, as long as PBN sites stay indexed, you're good.
To sum up the long list, IMO most people who own a small PBN, should just make sure to do the following:
- different hosting, IP, nameservers
- different registrar, whois information (privacy)
- different CMS (wp, html etc.) and theme / design / code
- niche relevant unique and readable content
- natural looking site with images / videos, subpages (contact)
- different anchors pointing to MS together with few different authority links
