Dashcam

I have been contemplating a dash cam for my cars for quite some time, and the article on Jalopnik about that asshole cop arresting a dude after the cop ran the stop sign has hardened my resolve to get it done.

I’m not necessarily asking for recommendations on cameras (but will welcome them) because there are plenty of posts on that already. I do, however, have some general installation questions.

Hardwire or battery? If battery, how often to they need to be charged?

If hardwire, how hard is it to do for somebody (like me) who has precious little experience with such things?

Are there shops what will do installs of equipment that I buy?

Do these things generally begin to record after an impact, or only when a button is pushed? (I understand the buffering process, like with a flight data recorder.)

Thanks!