Welcome to the forum
Well, I'm no expert on body lift surgery, but I would guess that there would be pros and cons of both approaches...
If you got the body lift and then went straight into building muscle after, that could cause the scar tissue to stretch. If you go the body lift after gaining muscle, your skin should "fit" but if you lost that muscle (like when you're older) then you may be left with more extra skin. I guess you should ask a plastic surgeon to find out for sure, but then again some may try and push surgery now rather than later for financial reasons.