Should I buy BTC on 6500, or wait to possibly go even lower?
That would already be a good price since we don't know how low will it fall. Setting an value to buy is a good idea. But still, that's up to you if you will wait for its value to be lower than 6500$ since whether lower or not on that value, the thing that matters is that, its market value will eventually increase so you better invest to it and hold.
Those who are waiting for the price of bitcoin to fall down now is the time to invest, don't wait for totally dump because we can never tell how the price will get low so invest while it is not yet too late to invest, and those who has bitcoin in their wallet just wait again for the price to go up don't panic it is just normal.