You mean there's a problem with static var? Inline value is just a magic constant without name and for me it is harder to reason about code locally.

I should use there private static. I could use instance fields if the kata was instance based, but that is not the case.

StringBuffer is way slower than StringBuilder, because it is synchronized. And if nowadays you see StringBuffer anywhere in single threaded code it is archaic (or poorly written) code.
BTW code from kata with +="" whould compile to StringBuilder concentrations, very inefficient ones however.