Downstate is farm country, and family farms can be worth millions to tens of millions on paper.

In my experience, the folks who have tens of millions of dollars in farmland are not concerned about the inheritance tax. They're incorporated in some fashion.

In my area, it is legitimately the folks who own 50-60 acres of ground that is worth $3000-5000/acre who are most concerned and vote on that issue.

What do the references "downstate conservatives" and "Donny" mean?

Donny is just an example name like Joe Shmoe

In Illinois, rich people live in Chicago ("upstate"), and tend to be more liberal, whereas rural folks downstate are conservative and less wealthy.

"Downstate" means south of Chicago (which is located in the NE region of Illinois).

As others have said, downstate means "south" Illinois. I'd add that it also means rural, instead of urban (Chicago, in up-state). The term "downstate" can't be universally applied to mean rural though; if we were discussing the same situation in New York, we'd talk about "upstate Donny".