For making your own flywheel steel should do fine. You're probably not aiming for an 80,000+RPM flywheel that rides on magnetic bearings. So composites shouldn't be needed. The best material, eventually, for flywheels should be carbon nanotubes. They have huge tensional strength, but lousy compression strength. Tensile loads is what a flywheel has, so they'd work great.
It is important to note that not all batteries associated with these computer models are affected. The company is directing all users to its HP Battery Recall website to go through the online validation and replacement process.
Update 8/02/2016 - Microsoft has announced a recall for faulty power cables included with the original Surface Pro as well as the Surface Pro 2 and Surface Pro 3. This recall does not apply to the Surface Pro 4, so there's no need to worry if you've bought Microsoft's latest hybrid or are tempted by it.