As I said there are already many threads with many good answers to your questions in this forum.
The first thing to do is to think about the size of the guitar, the scale length and the nut width. If your hands and fingers don't have average sizes/lengths, then you must clarify first if a guitar with standard sizes fits the best. Then the next question is whether you want to buy a new or used guitar. There is no general answer because both approaches have their (dis)advantages. An inexperienced buyer might get some help and find the right guitar in a local shop, but the shop assistants I have met in guitar shops only know e-guitars and can't really help with classical guitars. Going to a luthier would help but considering your $400 budget this is not an option in your case. Buying a used guitar online always have a risk. I bought almost all my guitars online and used, but I have seen many used guitars that are offered with a description that doesn't match the quality or state of the guitar (about 50%). Always contact a seller and ask for information or additional pictures before you buy a guitar (for example in the bay). Don't take recommendations for models or brands too seriously. If you ask 10 forum members then you get at least 11 different brands and models. Concentrate more on the properties of the guitar, wood types, which parts are solid, quality of the tuners, quality of the frets, string height, etc., and don't buy a guitar only because an "expert" has recommended it.
19 classical guitars, 1 digital piano - no TV, no radio