Reagan didn't "touch" medicare for the same reason no Republican president or congress will ever repeal Obamacare: by the time he took office it was already clear that the program worked well, and eliminating it would have been hugely unpopular. I don't know if Reagan had changed his personal opinion about Medicare by the time he took office. But when Medicare was proposed, he called it a socialist plot that would inevitably lead to the end of freedom in America. I gave you the direct quote above, and you can
listen to the speech here if you don't believe me. So, there is no argument to be had about "if" Reagan hated Medicare. When it was proposed, he argued that it would literally destroy the American way of life.
As for how Sanders would pay for his programs... Again, I am not a fan of Sanders. And I do think that he underestimates the budget cost of his proposed health insurance reforms, although not to the extent that all the major Republican candidates understate the budgetary implications of the huge tax breaks each of them (even Trump) proposes for the richest 1% of Americans.
That said, some of the items on your list, like increased spending on infrastructure and education, would probably pay for themselves in the long term. The pitiful state of our infrastructure, compared to that of other advanced countries, is a national disgrace at this point and a significant drag on our economy. So, increased infrastructure spending (especially when the government can borrow money at close to 0% interest) is a no brainer that would have already happened if our political system was less dysfunctional. And increased spending on education is also likely to be a good investment. The GI Bill (which funded free college education for US soldiers returning from WWII) was expensive, and also probably one of the best investments the country ever made in its economic future (Thanks again, Liberals!).
Beyond that, it's completely true that Sanders, who openly refers to himself as a democratic socialist, is advocating a shift toward more generous social welfare spending funded by higher taxes, along the lines of. . . pretty much every country in Western Europe. Now, you can pretend (as many conservatives like to) that France, Germany, the UK, etc. are unlivable socialist hell holes. Or you can just accept that that is a viable alternative economic model and argue that it is not the right approach for the US (which is a perfectly reasonable position). But you can't pretend those countries don't exist or that is "naive" to believe that it is possible to organize an economy that way.
