Most builders will honor your request to be represented by an agent if you mention it before you go there. Call the sales office and lay out the scenario, most often they are happy to have a represented buyer and will not exclude your agent of choice if you are upfront about it. As for getting pre-approved, talk to a lender and they can run your numbers, credit scores, job history, debt to income ratios, etc. No reason to look at a property if you are not qualifying for a mortgage in these tough underwriting lender days.
If you like my answer, best answer me and a thunbs up, Thanks!!