I have not found everything in steam store that i can see inside the store in the game. It is cheaper for me to buy in steam instead of the other store. For example i want to buy dtm2020 but this pack i can only find in game store
For lowest possible price grab vrp pack here: https://www.raceroomstore.com/shop_en/list_page?cat=24 and grab content directly from the game. I'd suggest grabbing packs, which lower the price even further. Most often advised is buying either premium pack or starter pack->xxx->premium pack. Keep in mind that having content from X pack, makes the price of X pack lower Steam store has some old dlcs and overall feels neglected- in 99,9% of cases it makes more sense to buy from within the game
The store in the game or on the website has all the content this game has. Steam only has several packs but they are subject to Steam sales and Steam's regional pricing. If not for the cars (i.e. you think they are old and/or rarely used), you can buy them for tracks, which depending on your region and discount might be a great deal. Buying content is like a game inside a game here. Consider all the options available for you and pick whatever is the best at any given moment.
And keep in mind, every pack in Raceroom store can be bought once. Especially premium pack is often subject of complaints because later added content has to be bought separately.