WOW! finally another kettlebell guy!!
yes, kettlebells are a great way to work the core as well as working the entire body as one solid peice, but if you dont want to totally commit yourself to kettelbell training there are 3 drills that specifically target the core in a way that NOTHING ELSE does better..trust me after these you are never going to want to do a stupid crunch again, dont go easy just because you hear crap about some actress or actor doing 1,000 crunches a day. I say train the core in a DIESEL way...... Double kettlebell windmill, turkish get ups and if you want to also work your lats in an unbelieveable way try a bent press. Now you add some kettlebell swings and that is one CORE workout not to be messed with.
PS: in all three of these not only do you hit your back, obliques, upper and lower abs however as a baseball player Im sure you wont mind the great shoulder flexability gained from the K-bell TGU,WINDMILL,BENTPRESS google these to learn how to do them
and yeah the 25-35lb would be good but if your strong dont be afraid to go biger on these