Games Workshop very very rarely will invalidate models. Every once in awhile (usually years) they will update some of the old sculpts for new shiny ones, but all the old models are still legal to play in all setting from casual to tournaments. A few models (usually older characters) will be put into "legends" which means they won't be getting new rules and are usually illegal in tournaments.
But honestly if you play a game with someone and they say you can't use some model, those people are not worth playing with.
As for what to buy. There is a motto of "buy what you think is cool, not what is good right now". This hobby takes a lot of time to build and paint so if your not interested in the things your painting it's most likely not going to turn out well. If you are willing to wait for the combat patrol box, they are usually a pretty good deal $ wise. If you don't want to wait and can get your hands on the 9th edition starter boxes, they have necrons vs space Marines and are some of the best deals for price. Never buy directly from Games Workshop unless you absolutely have to since you can find everything about 10-15% off elsewhere.
Hopefully that helps, and welcome!