Most of what you're looking for can be found on Google.
1. You don't need to buy a VPS for anonymity. The whole point of Tor is to be anonymous through the Tor network.
2. You don't need to buy an .onion domain. You don't pay for those. They are randomly generated.
Get Linux. Windows is not an option if you want to be anonymous (seriously, I know people who hold low-level jobs with Microsoft and even they can testify you barely have any anonymity with Windows).
Here's some tutorials I found that might be of use to you:
- http://www.makeuseof.com/tag/create-hidden-service-tor-site-set-anonymous-website-server/
- https://scottlinux.com/2013/10/11/how-to-create-a-tor-onion-site/