First of all: welcome back, I hope you will have a good time!
When you say "5th/6th edition back in the late 90s" I assume you are talking
WHF?
40K 5th/6th edition was a good ten years later.
In general: collect what you like building and painting. Most armies have a variety of playstyles, and chances are high there is at least some way you can play any of them in a way you like. So the building/painting part is more important to "get right" - as long as you value this part of the hobby.
In a casual setting things like "the meta" or "tier lists" do not really matter. By working together with the player(s) on the other side of the table you can make pretty much any matchup a good experience for everybody involved.
There is a new (10th) edition of
40K coming this summer - you can check
the first official news from GW here - so you might want to hold back on any rule purchases. What you could do: start with one of the
Combat Patrol boxes
GW does these days. Not only do they come with a reasonably varied starting army, there will also be a gaming format in 10th edition - aptly named "Combat Patrol" - where you can field the contents of any of these boxes against one another and have a balanced game.
Personally I do not think the
CSM Combat Patrol is particularly thematic for Night Lords, but it would fit great with Iron Warriors. What either of them get in terms of special rules - no idea as of now. Just keep in mind that the current Codex:
CSM will be useless once 10th edition drops.