Is mixed plumbing a red flag?

[George Kopp]

Somewhat odd question. I have a single family rental, which had updated plumbing about 20yrs ago, mostly copper, but the silly do it yourselfers have several occasions where that copper is touching steel duct work and other steel. If you know anything about metals, they don't love resting against other incompatible metals. I'm not a scientist (or a plumber), but I believe it's the steel that is corroding, not the copper. So maybe I can just put some plastic between them and call it a day, but my fear is that in the future the copper could leak, which could result in thousands of dollars in damage depending on when it's finally noticed. So my thought was to just replace that entire section with PEX. 

So here are my two questions:

- Is it the steel, which is corroding? If so, I think I'm okay.

- If you were looking at buying a house or investment, and you saw a random area of PEX, would it make you worry that the house is filled with plumbing issues and that what you see is just one visible problem amongst many problems hidden behind the walls?

That second question is my concern with this plan. I suppose another option is to sweat new copper and protect it from steel this time, but not sure that really addresses my concerns.

Thanks in advance!