I would propose JavaFX 8: It is a real easy to use and intuitive way to build the GUI, have a real fair amount of effects available and uses a proper MVC view.
It is not really hard to learn and also has a GUI builder available.
For the rest of the game I'd use Java 8 and then hook into some lower level API for the moddability, I've heard talks about Lua being a good choice.
Ultimately we should offer multiple options to download the game: One version could be a plain JAR, other one could be a natively packaged JAR which installs the game, and lastly we should have the default option to include the Java 8 JRE.
The coding itself could be done using Github and Travis CI, where the latter will take care of executing all unit and integration tests, and uploading every file to the correct place after a significant build. Both do cost some bit of money in case we develop it closed source.