Is it legal to buy expired domains with good DA and redirect them with 301 to ones sites for link juice? Does google go against or for it? Also what about seo?
sure it is legal but Google may notice that as well. Depends if Domain brings traffic what will be user experience. One thing you can do is to create some old url's from expired domain. Check https://archive.org/ for this then create unique content relative to topic
lets say if you will buy domain which is penalised and redirect pernamently,- what do you expect? Another example: if you will buy 100 expired domains and redirect them to your website? Maybe for some time you will get traffic and etc, but I do not think Google will reward you for this step. Soon or late will be action to prevent such blackhat method, because you try to manipulate search results. in summary one expired domain should not harm your site, just check all parameters...
Google cares about what looks natural and what is relevant with what your website should be all about. Of course Google has no control over the domains you buy. And having them at your disposal, no one can stop you from rerouting traffic from one of your domains to the other. That being said there are a lot of things that can go wrong and @moneyroad already outlined some of them. Domains with previous penalties, domains with irrelevant backlinks, adding too many backlinks at one time, creating a network of such domains - all those can be seen as unnatural and you might suffer because of that. That probably won't happen immediately, might even take some months, but once the search engines pick that up - you are gonna have a bad time. Don't worry so much about your links, content is the thing you should be much more focused on. Good content will inevitably bring you backlinks and those will all be relevant and earned.
I might be wrong but I think google has updated there algorithm to place little importance on expired domains.
It might work... till the next update... rather create separate site and start slowly building PBN. I believe the 301 is the wasteful option here.