I've had a lot of success using the FlexBuilder plugin for Eclipse. Also, you can just use the straight up open source flex compiler and write all your actionscript and mxml by hand using your favorite text editor.
Also, I would just use ActionScript and MXML instead of HTML/AJAX. Actionscript is basically just javascript anyhow, and MXML is a lot easier to work with than HTML.
Also, I would just use ActionScript and MXML instead of HTML/AJAX. Actionscript is basically just javascript anyhow, and MXML is a lot easier to work with than HTML.