Should Muller's investigation include Trump's financial history?
Not necessarily, but possibly. It depends on what information Mueller uncovers. If his team discovers behavior that indicates there is
actus rea on Trump's part, then the question will become one of whether the statute involved requires he and his team also establish
mens rea and the reason for its existence. If
mens rea is required, leads to it and its cause(s) may be found in Trump's tax return, in which case, the investigation should, indeed must, at the very least retrieve his tax returns along with other financial documentation that is either sworn or objectively verifiable. It's worth noting that
mens rea isn't generally established by attestation (barring the Moirai's favor), but rather by a preponderance of information that immutably, or nearly so, would lead any sage, sane and reasonable person to agree it exists.