Few questions

I'm getting into affiliate amazon program and planning to make niche website. My question is: is it better to choose lets say a broad niche like motorbike parts ( and than target longtail keywords, for example motorbike brakes, mirrors..) or should I narrow niche down to lets say sausage stuffers and similar keywords. Also is it important for seo that domain contain keyword parts?

It's a good question. I'd choose something that allows you to do both. Build a site that targets a few major keywords and build general authority to the site. Then add lots of great content to a blog that targets longtails.

Regarding domain, if you cant get an exact match domain for your main keyword then I wouldnt bother with near match. Instead, go for something you can brand.