Unfortunately, if you have not purchased the base game you won't be able to play custom matches. If you and your friend both purchase the base game you will be able to start a custom match and invite each other to play along with other people you t or with AI players
Yes, you will be able to play with all ingame functions in action. However, the board will always be fixed the same way with tiles and number tokens in the same spot, so you'll pretty much get bored and know where to place your settlements.
Just spend these 4 euros and save yourself some annoyance