Hi, there's no one-click solution because XML is not a specific format, it is a general specification and language that can be used in myriad ways.
Whatever tool you use, yes, you do need to learn some about XML to do this. Do you want valid or well-formed XML? Do you have a DTD, or a specification of desired tags from the CMS system?
When you say the text needs to keep its style, you understand that XML does not define any style except as applied to it by post-processing? You might tag a line of text as
<paragraph style="bodytext">
and then have an XSL style sheet or CSS recognize that attribute and process and display appropriately, but you aren't going to have XML that says "this is 9 point Times plain".
I'm sure this is confusing, but the fact is, if you can't answer the questions above, then you can't choose between avenue or XML Plus or any third-party XTension, because they have different capabilities and knowing what output you are after is essential to choosing the tool.
As for HTML, I can't offer any suggestions, so I'd suggest you look at the PowerXChange web site and maybe give them a call, and see what tools they suggest based on your needs.
http://www.powerxchange.com/