A Lenovo Thinkpad T430 is probably the sweet spot for those specs and that price range. Not a ton of good listings on Amazon, but there are options in the T430 out there where you could end up with 8-16gb ram, an i5 or possibly even an i7. Those old Thinkpads have withstood the test of time.
A good alternative are Dell Latitudes, maybe around the E64xx to E65xx series. Those tended to mirror some of the features, design, and robustness of a lot of the old Thinkpads.