Well, it depends about Niche nature but you can keep article length at least 600 words for any niche. However, you can check your competitors articles length and make a better decision about length. There are some niches where you can write 300 words articles.
1000+ words with 20% of keywords.
You have to be joking. 20% of 1000 is 200. You can’t seriously be suggesting anyone should insert that many keywords in an article?
It’s hard to say without analyzing the site. You should look at similar indicators of competitors for an approximate amount. Next, you need to assess the quality of links. Some links may not be equal.