Refinement: around the core concept of "Arcane Paladin", I'm grafting on some of the tropes found in Terry Brooks Druids from the Shanarra novels (especially Cogline) and Knights of the Word from Brooks' Word & Void novels. So the PC's main drive is still fighting evil by force of arms & arcana, but also the accumulation, protection and (selective) dissemination of knowledge.
At the moment, I'm thinking Marshal4/BttlSorc6. BttlSorc gives a WP, armored spellcasting, the celestial lance, d8HD, 3/4BAB; Marshal continues the BAB & HD, adding more WPs, helpful Auras, and all the Knowledge skills as class skills. That makes the Knowledge Devotion feat a source of many potential combat bonuses.
(Also in the mix, Marshal2/Duskblade2/BttleSorc6 or Duskblade4/BttlSorc6, for more spells at the cost of skill points.)
I'm still trying to figure out if I can do better, though. I've looked at several other PrCls*, and while many are thematically appropriate, many have prereqs I'll have difficulty meeting, or don't seem appreciably better than taking levels in BttlSorc due to smaller HD, lost skill points, lost spell levels, etc.
Tentative Feat Front-runners list (revised)
1) Celestial Sorcerer
2) Celestial Sorcerer Lance
3) Battle Caster
4) Arcane Strike
5) Knowledge Devotion
6) Celestial Sorcerer Lore
7) Celestial Bloodline
8) Celestial Sorcerer Aura
9) Good Devotion
10) Fiery Burst
11) Invisible Needles
* besides those already mentioned, I've also looked at the Kensai, Holy Scourge, Suel Archanamach, Mythic Exemplar, and Divine Crusader.