Here's where I think the "non commerce cannot be regulated" argument falls down.
I purchase broccoli. I don't purchase a lot of it, but I do purchase broccoli. I had some today in some Chinese food. I am engaging in commerce when I purchase broccoli. Furthermore, when I purchase that broccoli, very little of it comes from Michigan. It's interstate commerce. So, obviously, Congress does have the power to regulate my broccoli purchases under the commerce clause. No arguments so far, right?
Congress also has the power to make regulations that affect my purchase of broccoli, even if the thing they are regulating is not actually commerce. That's the essence of the Wickard decision. Congress can pass laws that substantially affect interstate commerce, including commerce in broccoli, even if the specifically regulated activity is not, in and of itself, commerce. I'll bet a lot of legal scholars didn't like the Wickard decision, but it is Supreme Court precedent.
So, what if Congress decided that making you buy broccoli was a good idea, and that somehow whatever benefit it would have would be because it affected the way I purchase broccoli, and that somehow my purchasing of broccoli was the government's business under the general welfare clause. I really don't know how convincing such an argument could be, but that isn't really important for the moment. The point is that Congress has taken an interest in my broccoli purchases, and they have decided that the best way to influence my purchase of broccoli would be to force you to purchase broccoli.
If that were to happen, then it would be clear that the "non-commerce can't be regulated" argument falls apart. They aren't regulating non-commerce. They are regulating the commerce that I am engaging in when I purchase broccoli. The specific regulatory action they take is to force you to purchase broccoli. Therefore, the commerce clause is applicable. Congress has taken steps to regulate my interstate broccoli transactions by ensuring that everyone participates in the broccoli market. That's what they are supposed to do under the commerce clause.
It's important to understand, though, that just because the commerce clause applies, that isn't sufficient to declare that the law be declared an acceptable exercise of congressional power. It is within their domain of things which can be regulated, but not every conceivable law that is in that domain would pass constitutional muster. Sure, it's interstate commerce, but is it something that is an appropriate exercise of judicial power. One could argue that any penalty for non-purchase of broccoli would be an unlawful deprivation of property under the theory of substantive due process. A court might be asked to rule that the broccoli mandate served no public interest.
In other words, it might be considered unconstitutional because it's none of the government's business to mandate broccoli purchase, but it doesn't make sense to say that any mandate that affects current commerce is actually not regulation of commerce.