how good is the emulation for these consoles ?
It depends on which card you use, see:
https://github.com/recalbox/recalbox-os/wiki/Board-Compatibility-(EN)
i was looking to buy a Pi4B but it doesn't supported. Should i buy a Pi3B+ instead ?
Sorry, but this is a decision that only you can make, really now Rpi4 is not yet supported, and there is no certain date for it to be, I cannot tell you "buy Rpi3 +", because when Rpi4 will be supported, you will hate me ... If you are in a hurry, buy Rpi3 +, if you are patient, wait to buy Rpi4 when it is supported.
will the recalbox automatically work with a wired joystick madcatz TE ? (It works on pc)
I don't have this joystick and I can't be sure, but search the list of compatible peripherals:
https://github.com/recalbox/recalbox-os/wiki/Compatibility-(EN)