To start, what you are calling an antenna isn’t like any other antenna you’ve ever worked with. Read this first;
You will have to build a matching L/C tank circuit … an inductor and capacitor pair that resonate at 125kHz… so you’ll likely have to either build / wind it yourself, or buy one of a known inductance and adjust your capacitance to match it properly. So, if you end up buying a 125kHz reader module with antenna already on board, and you want to trade out that antenna for another one, you’ll have to be sure the inductance of the new antenna matches that of the old one or it won’t work.
you should definitely look into a proxmark3 rdv4 with proxLF antenna… it’s the defacto standard device for doing any RFID research or tinkering.
not really… we ship all T5577 chips pre-configured with EM41xx emulated chip configurations, which is the absolute most common chip type read by basically every 125kHz OEM reader module.