Honestly, as a shielder the two most important pieces of gear is your shield and your weapon. The higher your shield skill goes the less important the other armor is since you'll presumably be absorbing most hits with the shield.
So go with something that you think looks nice.
I would recommend going blunt or pierce damage if you have 6 or less PS. Above that and swords are a more realistic option, looks like you're going that route.
I recommend the 21/15 build as shielder because it's just... really good. I'm currently going 24/12 which I love but 24/15 as you mentioned is also an option. I've never tried 21/18 as a shielder.
For shields Huscarls is an overall awesome shield that everyone uses.
Brown Lion Heater, Knightly Kite Shield, maybe even heavy kite shield are options as well.
Last thing I want to mention about weapons is the lower your athletics the more you might want to keep an eye out for weapon reach regarding what weapon you pick. With high athletics you can hug most anyone with a short weapon, but you might find yourself out of range and chancing more often if you go with a warhammer and low athletics.
