Hey Bigbadwolfi, welcome to the mortal realms!
I can help with a few of these questions.
Firstly, a really handy tool is the free
AoS app. It has the free rules, all the warscrolls for the units (their datasheets) for free, and then you can pay a really cheap subscription to build army lists in it, and can buy the Battletomes (codexes) really cheap within the app’s library store too.
I assume you’ll want to be playing with points, so if you and your friends want to, you can purchase the latest General’s handbook which has all the rules in it for matched play. The army Battletomes are also full of the usual stuff you’d find in a
40K codex too, I.e. relics, warlord traits, spell lores, faction/sub-faction rules, etc. but of course you don’t need to buy any of that to test out the game, as you can find the free core rules and datasheets in the app.
To answer some of your other questions about factions:
1) Cities of Sigmar is a really fun army. I’m not hugely knowledgeable about how good pistoliers are, but for 1k you’d probably be able to fit a couple of 5 man units or one 10 man unit in a 1k list and still have room for a General and a pair of Battleline choices.
2) If your friend loves Skellies and Vamps he should hold on before buying anything because the latest army
GW have announced and started showing off is updated Skellies and Vamps
I can’t really help with the other 2 questions, I don’t really have a lot of experience with those armies. I have heard a lot about beastmen being a little bad to play right now, unless you use a bunch of their Tzeentch units. That said, I’m sure they’d perform fine in a friendly environment, which seems the way with you guys if I had to guess.
Hopefully this helps! If you have any other specific questions or general things you wanna ask, or want things expanded on, let me know and I’ll see if I can help more.