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Firstt off, assuming the 700 is a FICO, you don't really have ok credit, you have pretty good credit in the world of auto financing. With that kind of DP, daler will have no issues getting you financed. I'd maybe recommend trying DCU before the dealer so they don't try to rip you off. As I recall DCU's rates are the same new or used (4.24%).
PenFed has lower rates (3.49%) assuming you are ok with a 36 month term, though PenFed will be harder to et approved for so I'd personally do DCU then ask dealer to beat the rate (if you don't care about inquiries).
I would say try your local CU, if you try through the dealer typically they require two years 1099 and 6 months worth of Bank Statements minimum but keep in mind too if you go through the Dealership preowned is harder to get approved than New was there a reason specifically you want preowned?!