Unfortunately there's no full plate with lower ACP, so maxing out parry (getting +25 skill check) means you will need at least +1 from equipment to max out parry. Without it, you're stuck at 24 (Skill Focus Parry and Improved Parry give +7 but ACP from full plate is -7 so without other considerations your maximum is 23 ranks +1 from capped Dex).
There are a few items that give Parry bonuses, but most aren't so good. Gloves of Swordplay give +3 Parry, for example, but that means forfeiting better items in your gloves slot.
Not sure it there exists a Great sword with +Parry. If there is, that'd be a good choice.