Being Berean using KJB verses NKJV - It is preference mostly.
The key thing to remember is the Footnotes in the NKJV.
And to have KJV close at hand. The continuity of the KJB as to rightly dividing the word is Key to the King James Bible. When NKJB changes thy and ye and Capitalization. It does change the flow of the Word and creates ambiguity in the sovereignty in Gods Word. To be a berean have the King James Bible handy and when You see footnotes READ the KJV and the Holy Spirit will guide you.
As for the many translations out there check which verses are eliminated and which are wholesale changed to meet copyright Laws. Remember Satan is the great deceiver. And his time is short. I expect many more so called translation to come. As to study bibles, there are many and the Holy Spirit needs to be your Guide. But the number one thing they must be based on the KJB or the NKJB. with the caveats mentioned above. May Holy Spirit Guide you as you practice being berean. Study the Scripture.