One would only ever say, "She needed us to pay," not, "She needed we to pay." As such, you would definitely write, "us."
As for grammar, if you want to clarify that by "us" you mean "the tax payers," then you would write, "She needed us, the tax payers, to pay." The phrase "the tax payers" is an appositive, so you would write it the way we write appositives, which is between two commas.
All of that said, there is precedent for writing, "She needed we the tax payers to pay." You may recall the Constitution, which quite famously states, "We the people." Since the people doesn't necessarily have to be an appositive, it follows that neither would the taxpayers.
Now, you may be thinking: But "we" in "we the people" is the subject, not the direct object? That's true. Nevertheless, there is precedent. The following article from Grammar Book gives a very thorough explanation of it: http://data.grammarbook.com/blog/pronouns/we-the-people-or/ .
In a nutshell, you could get away with writing, "She wanted we the taxpayers to pay." You may end up having to defend that grammar, but defendable it be.