I hope you had a good holiday!
To answer your questions:
I think you would learn a lot from doing a project like this. I must warn you, however, that competing at a high school level, many students do more original research. I don't know how competitive it is to reach "internationals" in your area , so it is difficult to say whether this will be a good project for you or not. You seem to be very interested in it, which is a huge advantage. If you don't like the subject matter then you won't enjoy doing the project. There are always possibilities for making your project more original and adding some creative things to it, these types of things normally happen while working on the project and are difficult to just magically think up. I have looked at the science buddies project ideas you gave me, and I have a general idea of what you want to do, but if you want to give me more specifics I could help you further.
Wikipedia is a great source to start looking for information, but the science and math sections are difficult to understand if you've never studied the material before. I would go to a library and get a thermodynamics/kinetics textbook that chemical engineers would use. We used "Thermodynamics, Statistical Thermodynamics, and Kinetics" by Thomas Engel and Philip Reid but it is NOT a good book and I would not recommend it (so stay away from it!!!
. A book I would recommend is: Introduction to Chemical Engineering Thermodynamics by J. M. Smith, Hendrick C Van Ness, and Michael Abbott. It is relatively dense but it has many things that would be useful to you. Research FUGACITY, which will help you tons. If you ask most people what "fugacity" is they will probably say "it has the units of pressure." Unfortunately not many classes really go into what it actually is ("Oh, there isn't a physical example for it" they'll tell you). After learning about it I can tell you that it is a mathematical term used to explain deviations from ideality. So, people essentially made it up to explain these deviations. It's a a term people use to help model a gas's behavior. Once you do some more research and understand the background information a bit better, you will better know how to model gas behavior. I'm not a huge expert in this area, I've only taken one class in it, but I can at least guide you in the general direction that would make sense to research.
As for a good place to get some data, go to NIST: http://webbook.nist.gov/chemistry/fluid/
At the above website you can look at graphs for certain temperature and pressure ranges. Play around with it a bit and see what happens when you change things.
For general properties about certain compounds, go here: http://webbook.nist.gov/chemistry/form-ser.html
For the main page go here: http://webbook.nist.gov/chemistry/
The two websites above were from the "Thermophysical Properties of Fluid Systems" and "Formula" (under the "Search Options") links.
I hope that helps!