One level of Dragonslayer grants you caster progression+bab+fear immunity, this is what i can think off the top of my head. 18 caster levels is hardly "not focusing on magic" though - you will get 9th level spells! I expect your BAB to suffer somewhat, i'll have to do the math though. D12 is great, and you will need to rely on spells for AC since you don't have any means to ignore ASF. But with spells like Greater Luminous Armor your Abjurant Champion class can really shine.
I'm not entirely sure dragonslayer would be thematically appropriate for someone who is slowly becoming a half-Dragon. >.> Also, my monk stuff grants Cha to AC... so... by pumping my CHA for DC's and the like, I'm also increasing my AC.
And Abjurant champion was definite meant to shore up Aidan's inability to wear armor... since he's supposed to be able to mix it up in melee. But, I definitely need to get Aidan's BAB up... since I need +5 for AB, and I have about 3 levels to get from 2 BAB to five. I'm thinking I may need to sacrifice another level of spellcasting for a full BAB class.