After the Starter, What to buy next?

Yes! You can stick with the models you like. For expanding, I would recommend making sure your first 12 models are 2 of each type. The starter comes with a Guard, Rogue, Gunner, Scout and Specialist, so you'll want one more of each plus two fighters.

For now the game is very casual, but they definitely have an eye for competitive play in the future. Honestly, even at it's most competitive, the game is casual enough that everyone is on a pretty equal footing thanks to the dice swings.

The current recommended way to play competitively is the Adventuring Party format. Each player brings a 6-man Crew with 1 rep from each class. Hence the recommendation to get two of each class so you can introduce someone to the game quickly while also giving yourself plenty of Crew compositions for Adventuring Party.

Best of luck!

Frankly, the way the game works makes it kind of hard to make a "bad" hero. Thus far Gubbins is the only hero I've ever regretted taking, because his two health makes it possible for an enemy to take him out in one shot and I keep finding that he converts into an easy VP for my opponents a little too often. Wolf is a great Scout character, less mobile than Eiryss but he can fight at range kind of like a Fighter. I'd say if you take him you might want to look at getting a mobile character in one of the other classes, Black Bella or Dreyfus for example, or if you're happy to double up on a class then he really likes Eiryss for company.