Re: Lets say I can only afford 1 kettle bell
Might depend on the source of your lower back pain if you want to do swings mate; as you probably know they're taxing on the lower back and a great way to strengthen the muscle there. If you have damage there you probably need to start lighter and work up, but if the pain you experience is down to a need to develop the muscle there you could start heavier and the swings will have a great benefit for your squats and deads over time.
My gut feel for just one bell based on your strength level would be a 24kg. I do 3 x 24 rep swing sets with a 24kg bell with 8 one hand, switching straight into 8 with the other and 8 two-handed. It's a great lower back and hamstring developer. 24kg is also a good weight to shoulder press, goblet squat, clean and press etc. A 20kg would likely be a better starter but I'd suspect you might be wanting the 24kg after a couple of months. That said if you're happy to just profess to higher reps over the longer term the 20kg would probably be just as ideal.