Welcome back to Dakka and
40k.
The new edition drops tomorrow, so things are a bit in flux right now.
GW has released a lot of the rules and such, but I know personally nothing really gels until I have the books in my hand.
Downloads:
https://www.warhammer-community.com/en-gb/downloads/warhammer-40000/
Getting the official
40k app on your phone will help a lot, but is not technically required.
You will probably need a mission deck. I’m not sure if each player needs their own, or if one will do. Or if you just use the app for them. There is a new feature where you can set up battles, track stuff in game, link to your opponents list/rules, but I’ve not plaid with it yet.
Objectives are now bits of terrain, and map layouts are set (via the mission deck) you can find the templates in the
misc section of the downloads if you want to make your own templates.
So from a rules POV, the core rule are free, but you will need your codex, the
DA supplement, and maybe a mission deck. Marine codexes are normally early in the edition, so I’d not drop the cash on the 10th ed one to start. 3rd party list builders like New Recruit will give you enough of the basics to push models around, so can act like a stop gap until the new book drops.
To play you need dice an a tape measure. More then that there are things are are nice to have, but not required. If you are setting up your own table you will need the terrain templates, and some clutter to put on them. Could just be soda cans and scraps of cardboard, we all started somewhere.
What minis to add to your collection is a very complicated and personal question, with no single answer. Especially around an edition change.
GW has been pretty good about providing legends rules for minis they are retiring, and those should be fine for non-tournament play. “Safe” minis are going to be primaris minis released in the last few editions. And nothing is 100% safe. Some newer models that were released on shared sprues might be on the chopping block. We really don’t know.
https://wh40k.lexicanum.com/wiki/Portal:Miniatures
is a nice list of when stuff was released.
At this point I would not recommend picking up older firstborn kits. Some might get recut into new primaris versions, or quietly sunsetted. We don’t know.
What units do you like? My general army building advice is to find the 2-3 units that drew you to the army and get them. Then figure out what support they need to do their job. And then add other units to fill the roles you need to win the game.
(Full disclosure, I play Ultramarines and prefer a balanced, take all comers play style. This is not the only way)
There are roles that generally always need to be done. Tanks need to be blown up. Hordes of weedy troops or more rugged marines will need to be mowed down. Boots will need to cross the battlefield and stand on objectives. Then there are more intangible things. Sometimes the job is to take up space on the battlefield to screen your more valuable units. Rapidly reposition to another sector of the table. Sit still and spend a turn taking an action to generate victory points.
Marines have a bloated codex. We have a TON of units that can get the jobs done.
One thing from 10th and going into 11th is detachments. They tend to give specific units and playstyles extra buffs/tools. You are generally going to want to pick one that matches how you want to play. We will get 3 points to spend on them for a 2k list. I’ve not dug into them yet, or looked at the
DA ones. But for army building, they are going to be important. In 11th you also need to pick a play style from your detachments and when you compare it to what your opponent has will dictate the mission you play. So if you are playing a kill everything world eaters army, your win conditions will always reflect carnage. While a tricky Eldar army might be spending more time getting
VPs by doing esoteric actions in their enemies deployment zone. In theory. This is one of the things I need to sit down and ponder once I have the whole picture in my hand.
Another thing to consider is that characters generally attach to units. (not the Lion; he just runs around on his own) So consider what they bring to a unit, and what synergy can be found there. Not that you need to min/max everything, but if you have a
TDA Librarian, you probably want a squad to stick him in. They all have restrictions of who they can join, so I’d suggest looking into a list builder just to noodle ideas around in.
https://mfm.warhammer-community.com/en
All the points are here and will tell you who can join what. Not the details (like your
TDA librarian will give sustained hits to his unit) but just the
raw facts.
I could ramble on more, but dinner is not going to cook itself. Hopefully this helped.
I have been trying to get back into the game for a great many years, but life.
6400 pts+ 8th
Ultramarines, 3rd Co. and friends, 16k+ 
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