If you want to do 16-bit, get a used Sony SBM-1. There have been (and may still be) a couple floating around the Yard Sale section. It is an A/D, but will make your recordings sound good. I also used a Denecke AD-20 and liked that a lot. Combination preamp and A/D. Both units are very stealthable and fit in your price range.
I have no idea of what to use if you want to do 24-bit in that budget.