I am planning on buying a 2003 Chevy Silverado 3500 and I was wondering if anyone knows all the vehicle fitments because some say 2500HD-3500, 2500HD-3500HD, 2500HD. I just wonder if they would all fit the same because some say yes and some say no.

From what I understand, the 3500hd is the same with 3500 for the current models. About the lift kit, try to ask the manufacturer to confirm this. You can also ask some lift kit shops like Suspension Lift Kits|4WheelOnline and ask them about the available lift kits for your rig.