KT Newbie questions

Greetings!

Me and a friend are old 40K players returning to the hobby and in order to start again we decided to gift leftover minis & abandoned projects to four more friends, thus creating our own gaming group. We plan to play narrative campaigns (with rpg elements) and decided to organize painting days when we will all collectively paint an equal amount of points and progress our army building together. I will build death guard, friend 1 blood angels, friend 2 space wolves, friend 3 Grey Knights, friend 4 Chaos Space Marines and friend 5 Custodes.

We decided to go for the kill team -> 500pt combat patrol -> 1000pt army ->1500pt army -> 2000pts army route.

I was today years old when I learned that Grey Knights and Custodes are going to no longer be supported by KT (and the SW friend isn't really happy that he has to use generic SM units either).

The good thing is that we are not bound by tournament rules and we can collectively make tweaks (friend 3's Long Fangs are staying despite not being in the new codex and his generic SM entries will have the SW keyword).

My questions are the following:

1. Should we play the newest edition and convert the compedium entries to that, or stick with the previous one?

2. Are there specific terrain maps to use? The GK friend is a terrain-making freak, so are there any guidelines of how much terrain is needed?

3. Which books do we really need? I am confused with all those supplements, years of release etc.

Thanks in advance!