Newbie here got a question!

So I've painted warhammer 40k minis now for 7 years, and guess what I've never played once EVER lol crazy isn't it! Im an artist who dropped digital painting to paint minis. I recently saw that kill teams are a thing and im wondering how complex is the game is self? Is it quick battles? I wanna get two armies for me and my uncle (he doesn't know much about warhammer but loves risk!)

Should I just pick a faction i think is cool and go with it or is there some new edition i need to buy to be relevant