use Youtube - free and has some very good videos on gas turbines, piston engines, gyros etc.
I wouldn't buy additional cbt unless it is going to be part of your chosen course. Some cd's are available for free download (not that i'd condone that, obviously)
there's only so much study material you need for these exams, i wouldn't worry about getting lots of different material as it will just increase your workload. Just make sure the material is up to date.
Good luck!