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The difficulty of home weight set ups is that even if you are strong as fuck, upper body stuff is going to use much lighter weights than lower body.
Up to 20kg could be a good start, but might be worth doubling up on some of the higher weight ones for lower body work. Can always buy more later.
Might be worth just getting a day pass at a local gym, having an idea of the movements you want to do with the kettlebells, and seeing where you're at with them, as a guideline for what stuff to start with. Can always buy heavier ones when you're stronger.
Next question
For my wife and myself what sizes of kettle bells should we buy? We aren’t looking to become weightlifters, just to maintain functional strength as we get older so we can continue cycling, swimming etc.