I think you can use wget, if you need to download something outside of the package manager, and you should be able to use the package manager (apt, apt-get, aptitude) to install things that are available in the repositories. You can manually add repositories in /etc/apt/sources.list , if that isn't set up yet.
If you mean "desktop environment", instead of "nice file manager", look into XFCE, LXDE, and GNOME. If you really meant "file manager", I can only recommend Thunar, because I don't use anything else.
The usual "graphic desktop environment" options come with a browser, and everything. (If you install the XFCE or GNOME "solutions", you will get a browser too, I believe.)
Anyway I hope this helps you on your way.