a) I am not an expert in similarweb.com. I suppose they base their data on statistics (# of links, alexa rankling, # of social signals, DA/PA ...)
Like any statistical method, the more data you have, the better your prediction is. In other words, similarweb is probably more accurate for big sites than it is for small sites.
May be a better tool is semrush.com
They periodically scan the results of G for tens of millions of keywords known to have traffic. They store the top ~30 results (SERP). For a given domain, they are hence able to find which keyword ranks and what traffic it brings. semrush.com is very well regarded in the seo community and their results are probably more accurate. However, they are pricey and probably you cannot afford them for what you have in mind. Of course, this method will says nothing about non-seo traffic.
Side question:
There are may be 300k expiring domains each day. If you flood similarweb.com with that many requests, you will soon be banned. Do you have a pre-selection process ?
b) By high DA, I rather mean DA 50 to 70 or more
Domains with such DA are not in expired domains of course, but it's possible to find domains having links from such high DA.
The main idea is to pick up high DA domains (say: wikipedia, business week, ny times...), scrap all their pages and check on each page if there are dead links to expired domains. You can do that with utilities such as xenu. I know some companies offer this as a service (I remember Terry Kyle does that, but there are certainly others).
To be honest, I am dubitous on the idea of doing that for CPA monetization. My idea was to resurect or re-create web sites with targeted traffic by using high DA links.. I would then get an authority site with targeted traffic and flip it. I am not yet operational. Just sharing ideas (or dreams).