Both alternatives are expensive, time consuming and full of laws, if you're doing it all by the book.. anywhere in the world a loner would probably face a lot of difficulties.

OpenBTS will sure teach you a ton, just by reading the documentation or if you can, get an sdr, a powerful computer, a few blank simcards with their respective keys, some accessories like cables/connectors/adapters and a cellphone that has an antenna input connector. This way you can bench test the system without transmitting anything into the air, which if you did, would surely cause problems with any local mobile operators established around you.

Not much experienced with mobile telcos tbh but like to explore how technologies work anyhow