I have a laptop with an integrated AMD gpu that is about as good as current Intel integrated gpus and I'd recommend against developing 3D games with it. Even in fairly simple examples like the 3D platformer demo, framerates are very low when running from the editor and still less than 30 fps even in the exported version (it is possible for a game to be simpler than this demo, it just depends on what you're doing). Maybe you can just purchase the laptop and return it if you can't run some of the Godot 3D demos, depending on the return policy. A discrete gpu is definitely worth it though, even a very low end one or one from 2 or 3 years ago. As for your list of things you want the game to have, only things that have vertices and things like lighting/shaders will have a significant effect on smoothness of gameplay.