1) no , you will get the source code , and it's up to you to compile it , and set it up , also you wont get a ready to play client , you would have to do this by your self too , however if you follow the guides in the forum , excluding the download time , you can set up a local server in less then 10 min , everything depend on your experience
2) yes , you have to install some kind of sql
3) Always/Never , well , it's hard to tell , you see the updates , if it's a bug , and it would effect your server , you would want to apply it to your server , right ? if it's a public server , you should use git commands always , and you can update your server like once a week , of once a month , the longer the time you wait the more updates you would have to apply, I wouldn't recommend more then a month , however if your server is fully customized , I wouldn't suggest a normal update, in that case , 1st i would suggest to try to get your copy as stabile as possible , by applying fixes only , you would reach a point where most of the fixes on rathena for things that you didn't apply to your server anyway
4) it's a database, so it's very easy
again everything depend on your experience , everything could take around 10 minutes or 10 weeks ?
I would just suggest to follow rathena wiki first , and if there is anything you didn't understand , or you are stuck , you can search the forum.
Home · rathena/rathena Wiki (github.com)