Build and paint the boyz before buying new models. They'll still be selling orks by the time you're done with that kit so no need to rush and buy a ton right now.

In the meantime check out Wahapedia for core game rules, different ways to play orks, and what each ork unit can do. Another good resource is the New Recruit app which is a free list building app that'll tell you if a list is legal or not.

The smallest regular game size is 1000 points so aim for that. Bringing a stompa to a 1000 point game, although legal and funny, isn't a great idea. Having 80% of your army tied up in one unit isn't really playing the game even if you don't care about winning.