Yes and no. Transistors are already smaller than blood cells. Powering the transistors are quite tricky; they could run using biological engines.
However, it's quite difficult to build machines to detect "cancer cell vs normal blood cell" at a nano scale.
I agree with this. Even if curing cancer could be made possible by nanobots, only the wealthy would be able to afford it as I expect it to be extremely expensive. Especially if it will garner 100% cure, these nanobot treatment won't come cheap I'm sure. And I agree with the Jet Cash, it might be better if they focus more on maintaining healthy immune systems. Not that this isn't a great idea. But using it as preventive measure will be more beneficial especially to families that have history of cancer.