My suggestion in buying KBs to progress:
Start with one, like a 35#. Work on that until bored or it's too light.
Then go to a heavier single KB, like a 44# or 53#. Same thing.
Then get another 35# and start working double KB exercises.
This way, in terms of the weight your posterior chain experiences, you are progressing in a very methodical manner--35#, 53#, then 70# (2 x 35#). If you are buying them piecemeal, that's the way to go (that's what I'm doing now, I sold all of my old doubles of DD KBs so I could get the new GS-style KBs. My first purchase will be a 44#.