verifiable research statement
If you're just going to talk to professors ahead of the application, you don't need to bring a portfolio and certainly don't need to bring letters. That's just overkill. 

Just come prepared to talk about your work. If you have any publications that came out of it, you could bring those. Or even a brief summary sheet. 

Verification will come later when the letters of reference come in with your entire application package. 

Use your early visits to get a sense of the departments and the faculty who might be enthusiastic about your project and possibly serve as a PI. You'll impress them most by being enthusiastic about your work, being able to articulate its importance without having to fumble through your briefcase for external validation, and being able to express great interest in working in that particular discipline with that particular department and PI.