Regarding the second part of your question, I think you'll be fairly safe as long as you follow a few simple rules:
- Install a good adblocker.
- Use a separate anti-virus/firewall package and keep this up to date.
- Don't click on any suspicious links or strange websites.
As has been mentioned, security updates will continue to be made available, but this will trail off and you may not have access to all the newest bells and whistles, but if you can live without that it is certainly an option to stay on Win 10. I have a set of in-laws who are still on Win 7. The challenge of converting them to a newer Windows version is too much hassle compared to the risks they're running. They know not to open any suspicious e-mails, they have nothing on that computer that could do them any damage as they don't use internet banking at all and have never had a credit card. Still going strong after several years like this!