id sujest if ur worried about your knees. why dont you get a personal trainer to train you, that way they can give you a solid analisis of how far to push and what your ready for.
btw your first question was funny . the one about working upper body and getting gains without working lower body, it funny cause 80% of the guys in my gym dont work lower body cause ' they dont want there legs to be huge like those body builders' hahaha