how about this coin development recently?If I want to create a node,how many coin should I need?
250k Stratis per Masternode.
Info about other nodes has not yet been released.
Stated in Q&A that people holding just 1000 STRAT might be able to create small nodes, lets see what comes.
To operate a node i have to be online 24/7 right? What happens when the power goes out tho? Or I get a bluescreen while being on vacation?

You set up a remote connection for your computer and keep tabs on it. I can access my pc at home from my phone and relaunch any miners or wallets that need syncing.
Chrome remote desktop is easy as pie to set up.
Wont that make your wallet less secure?
Depends on the other safeguards you have in place, which is another subject.
The solution is just that simple is all I'm saying, the rest is your responsibility.
At a basic level, it's secure enough if you don't do anything questionable with your browsing habits and keep
your computer secure of viruses and what not.