Oh yeah sorry I didn't even think of that, the actual map customization is non existent. It's just the options. It's not ideal but I think they made some good maps anyways though.
By all means do the campaign if you want I'm just telling you the sandbox is good too. Campaign will definitely put you into different situations that will teach you the details of the game. Like having to be successful with tourism, it's not very easy compared to just mining or using factories.
I just play sandbox mode usually. In that the easy maps are extremely easy but the hard maps really test you. I played one the other day with no metal in the ground, no oil and almost no good land for farming. Making money isn't so easy, to put it mildly.
20 isn't bad at all really. The core game is 29.99 on Steam, on sale for 10 but I think the version you got is more like the collectors edition with all the DLC right? Sounds about even.