Old player coming back to game, where do I start with squad building?

Hi, I'd look at building up your CLS and Rebels initially. It's still an excellent squad. Another change to the game is a Conquest mode and CLS can do some work for you there.

I'd also bump up Padme and her Galactic Republic friends - she's also helpful for Conquest.

If you're thinking about going for a Galactic Legend then Supreme Leader Kylo Ren is a good place to start. You get two squads and some useful ships to work towards a First Order fleet.