Fix Your Knob Bro
You should be concerned about every single detail about your home. Doorknobs are no exception to that rule. If you're not working with new knobs or used knobs that you love, then what are you even doing?
This One Can Be Put Off
Knobs can be an afterthought, as long as you're thinking about them in general. If you can open the door, then waiting until you have sufficient funds, or waiting until you find the exact doorknobs that you love, is something that you should do. You should also prioritize getting your doorknobs second-hand!