All you need to know about why the District Court decision in Virginia striking down the individual mandate is wrong is contained in this single sentence:
If a person’s decision not to purchase health insurance at a particular time does not constitute the type of economic activity subject to regulation under the Commerce Clause, then logically an attempt to enforce such a provision under the Necessary and Proper clause is equally offensive to the Constitution.
Judge Hudson doesn’t cite to any authority for this remarkable proposition, because there isn’t any. The very idea of the Necessary and Proper Clause is that it permits Congress to do things that are not authorized under its enumerated powers if they are rationally related to doing things that are, so long as they are not prohibited by some other provision in the Constitution. So it is simply fallacious to claim that if the individual mandate is not itself a valid exercise of the commerce power, it cannot be permitted under the Constitution because it is necessary and proper to something else that is.
Because Judge Hudson collapses the Necessary and Proper Clause inquiry with the question of whether the individual mandate is itself a valid exercise of the commerce power, he does not even consider its relation to the guaranteed issue requirement of the ACA. That requirement is clearly a valid exercise of the commerce power, and absent some other prohibition in the Constitution, the individual mandate is plainly necessary to its implementation.
Instead, Judge Hudson concludes that the individual mandate is prohibited by the Commerce Clause because it is not permitted. In so doing, he reads the Necessary and Proper clause right out of the Constitution.