I play basketball and am probably way too bulky...so here are my 2 cents from the other side of the spectrum:
- Strong glutes and hamstrings I think are obviously necessary. I don't think an overly muscular lower body can hurt in basketball aside from making it difficult to go over screens.
- It's important to be able to drive to the hole, protect the ball, get hacked and maintain enough balance to put up a good shot and if necessary be able to instantly reposition yourself for an offensive rebound. For this, shoulder strength (being able to keep your arms going up for the shot despite their arms coming down on yours while filing or swiping for the ball) and forearm strength (so they can't strip it from you) are important as well as core strength so that you can maintain your upright position and square yourself decently when pushed, bumped, etc.
- As far as how much is too much, upper body wise, I'd say you've become large enough or possibly too large when it becomes difficult for you to easily split a double team.
- Not sure if it's a concern but I wouldn't worry too much about additional bulk hurting my vert progression. Realistically, unless you're 6'6+ and freakishly athletic you're never really going to be jumping over people in meaningful games against players at your skill level. Learning how to play smart, reduce ground contact time and being strong enough to absorb contact and remain in control is much more important IMO than adding a few inches to your vert if your goal is to do well at basketball and not just post dunks on youtube.
- I'm in the minority here but I think the broad jump (e.g. jump stops, playing the passing lanes) is of more benefit for basketball than the vertical.
