The campaign mode has been removed from Discovery mod. It's more of a sandbox made for people to test ships and other things without spending money on the actual server.
Yes, you can buy destroyers in the singleplayer and no, there are no transport ships with 5k+ cargo
What unstable portals? Are you talking about junpholes?
You installed the normal version of Freelancer at some point. That is the one you need to launch to play the vanilla campaign. Vanilla means the original game. You can't play the campaign with Discovery mod. We disabled that.