Looks like your current problem isn't about choosing a bike, but rather convincing your parents that a used 650 kawasaki is suitable for you.

Maybe bring a mechanic friend or your brother to check the 650 kawasaki you are buying.
Can also say my money my problem

Also keep in mind that buying used bike, you need to prepare spare cash in case any breakdown happens after you buy it. Fork leaking, electrical problem like coil/rectifier, if you are unlucky will cost you a bit.
So you might also need to calculate how much you have after the downpayment.

Compared to buying new, it is covered by warranty.

reliability wise, kawasaki > cfmoto any time.
just because its used, doesnt mean its unusable.

have you tested either the z250 or z650? make sure to do so you get the feel of the bike if it fits you