i think shop tracks is a bit diferent i think it will ruin a bit the game cause you having to buy things just to make tracks would be a problem to custom tracks creator and for kodub mainly(cause he use a huge amount of things to build)
There is no plan for an in-game currency right now.
I don't want to make it so you have to buy track parts.
There is currently no limit for the track parts you can have in a track. But there might be some kind of limit for tracks uploaded in-game in the future.