SC2Mapster

You'll need to read the triggers and such in-depth. Units should be available if you use HotS Dependency with Campaign content on. As for tutorials, there is a Sc2Mapster wiki, https://sc2mapster.gamepedia.com/SC2Mapster_Wiki, and it has some tutorials. Most of them should still work, but they may not be what you want. The SC2Mapster discord can also help, as it's quite active.

Use Open Maps then go to Campaigns. If you own HotS then you should have it. Might need to go down the maps, but HotS and LotV are in their own folders.

They might use abilities, but I'm not sure on that.
New regions can be done via regions and the Set Playable Area trigger I believe. 
You can have star-bases with units on it now however with the water pathing, but for now just focus on the attack lanes.