1-stalling is NOT normal in any gear. your 2007 uses the EFI first series electronics package so the curb idle speed can be adjusted manually. typically what resolves low speed stalling on your year XB is a tps reset to value # 5.1 along with careful adjustment of the warm motor idle speed to approx. 1000rpm. you'll need ecm access hardware to do all this so suggest you either start educating yourself on same OR bring it by my shop and i'll set it up for free for you.....southern PA.
2-used parts can be found on this forum from board members....ebay...several other buell specific internet sites.
3-buell XB's do not have "rear-sets". what you're referring to are the rear L&R passenger peg stirrups and peg assemblies. you'll also need the mounting hardware. use #2 above to source same.
4-custom levers and frame sliders? ask the young punks on here. i'm a stocker. WARNING: be damn careful what levers you install. many a clutch system and front brake pads and rotors have been destroyed by amateurs installing incorrect levers. consider yourself warned.