The way CS does content updates, part of the update is free and part of it is paid. So, organically the best way to figure out what to pay is to play the game, see which features you like, and get the DLCs that expand the areas of the game you like the most. (I also recommend this method because if it turns out you dont like CS after all you arent too badly in the hole)

They do occasionally go on sale, so you can always just wishlist something and come back later.

The DLCs add stuff, but you don't really need them to start out. The base game is great as-is.

Other than that, you can see what's included with every DLC on the C:S wiki: https://skylines.paradoxwikis.com/Downloadable_content. That way, you can decide which DLCs you care about and which you don't.

If you have Mods you don't even need ANY DLC, really, though the park ones make the "park problem" completely a non issue.

Mass Transit is a joke, all the options are just memes.