page authority and domain authority are a good measure of how good a website is (but not the only measure!). Think about where your post and link are going to be in the long term. For example, if you are guest posting, the post may be on the home page for a week or 2 but then it will be archived so you need to look the inner pages of a website and look at the authority of that page.

If a website has a much higher page authority than domain authority then it's a sign to me that it isn't that good. It'd be much better to have a higher domain authority than page authority because it will show that the website has been up and running for a while.

Also look at the last time Google cached the website and the inner pages, that way you will see how often Google revisits the website and the chances of your link getting cached. Look at some popular websites and look at the domain vs page authority scores, the home page maybe slightly bigger but not that much.

Hope that helps!

I'd say a product page (if it is very similar to the product that is no longer available). If you have something like "You May Also Like" on the products this would be useful to have. Only my opinion though and there is no right or wrong answer.

I'd usually say redirect it to a category page (zip code or city) straight away but from a users point of view it may not be the best thing to do. Maybe leave the page up with a sold sign and then have a "you may also like" section underneath so they know the house is gone but there are some other options, probably the best for UX. After a few weeks, I'd then 301 it to a category page.

Google still crawls and indexes "hidden content" but from a user point of view if you have to hide it then it probably isn't worth reading. From experience, we use some css to dropdown extra content when the client insists on it being there but it is too long.

If it has to be there then I don't think it will be doing any harm but if you don't need it then get rid.

What content is going to be duplicated? The reviews?

The best way would be to add a canonical tag to the website. You need to identify the main page the content is going to be on then add the canonical tag. For example, If you have 4 pages with similar content then pick the page that has more authority and canonicalise the others to that page.

Here's more info on the canonical tag https://support.google.com/webmasters/answer/139066?hl=en