Can it open a textfile, read its content, parse it, and perform actions based on that?
Lua has a file I/O package, yes, and I will enable all the default Lua packages.
Can it also get information FROM the tracker?
Yes, that is the entire point of the API.

Oh, and if possible, can the script generate a dialog with buttons and perform actions based on custom button presses?
Oh come on, it's even being discussed in the first post on this page...