What version do I get?

If I decide to buy the Deluxe Premium tomorrow through Steam, what version will I get?
Is it a new version WITH the patch included or
Is it the “old” one and then I have to download the patch afterwards and then update the program?

You will install the latest version of Microsoft Flight Simulator, which is currently

Even if it isn’t the new patched version the patch isn’t that large or hard to install. After you download and install the game it will start and on the checking for updates screen it will see the update and walk you through getting it.

However I would assume you will get the latest version if you purchase it at this point.