1) Depending if you use Evoq or DNN Platform. Evoq is the commercial product by DNN Corp., and costs an annual fee. DNN Platform is the underlying Open Source Product maintained by the community, free and published under the MIT License. I have never used Evoq, but I realise that more and more people try to switch from there to DNN Platform, which could be a challenging task. But there are people around here who have done this multiple times, and therefore have some experience with it.

2) Can't say anything, as I don't know Xara. DNN is built on ASP.Net Web Forms, but also supports ASP.Net MVC. The content is stored in a (MS SQL Server) database, so it relies completely on Microsoft technology. Normally, you enter the content directly, and it is published immediately, but there are also modules that support publishing workflows, and there is also the possibility to use staging servers for the publishing workflow (what I never used personally).

The community here is nice and helpful, so feel free to post any questions you have - I am sure that somebody knows the answer and is not hiding his knowledge...

Happy DNNing!
Michael

Xara is a basic website builder that you can compare with Squarespace, Wix etx.
DNN is a CMS that allows you also to build applications. So, a different animal.

If you are just working with content, you don't need to download anything, just login and start working.
Monthly costs are limited the hosting costs (unless you have the paid version as Michael mentioned but I don't think so). If you are not the one hosting yourself: 2 names that come up the most are DNN4less and Managed.com