I don't really have any list advice for you but I'll give you some general hobby advice.
Don't buy stuff because it's the new hotness in a competitive sense. 1) That is almost always fleeting. 2) If you have a super powerful broken unit and your buddy who's getting into
40k with you has nothing of the kind, it's kind of a violation of the gamer's social contract. Now, if you decide together to bring the heat, by all means.
Do buy the stuff you think looks cool, no matter how bad it is. The Tau are lucky. We're one of the most powerful books, and we're not mono-build. Hell if you love Vespid, make them work. I'm sure it's possible.
Depending on how talented an artist you are, go slow on the painting. I'm NOT a talented painter at all. It took an airbrush, and a lot of practice with it, to get my force to look decent. If you're kind of like me, practice on some cheap terrain first. Experiment on test models, multiple times if necessary, before you decide on a scheme you like and a method that will make things look consistent.
You're coming in at a good time. 8th edition has many of us optimistic.