One of the best CAM packages for beginners that I have come across is FeatureCAM. Its pretty straightforward and pretty powerful once you get good at it. You can check it out and download an evaluation version @
www.featurecam.com Unfortunately, while you can make programs and simulate, you cannot access the G code. If you try it and like it though you can buy it. Its not cheap though. As for G code specific to Haas machines, there are only slight variations between the codes for different machines, and you can easily set up your software with postprocessor plug-ins that are specific to a certain type of machine, or controller i.e. fanuc or whatever. I think the evaluation version of featurecam comes with a ton of postprocessors, your mini mill will be in there.
Good Luck