First: The DM will have someone assault your building. It's too big and obvious for that not to happen.
To deal with that:
-Get a bard follower who can play a lyre of building. 30 minutes of complete invulnerability for your castle will help a lot
-Make your castle flying. Because it's far more difficult to assault, and takes off 5% of the cost. Add in teleportation 2/day, and never worry about ambushes again.
-Look in the DMG II for spell turrets. Because they're pretty solid, even at high levels, and can easily be used for utility (buff 'n stuff)
-If possible, get an ice weird or snow weird or a similar creature to take up residence there. Forewarned is forearmed, and unlimited scrying rocks.
Second: Those mantles as a whole don't look that useful, and you can get screwed over quite easily on the ones that are. For example, you can pass a Paladin of Honesty a counterfeit bill made out of cardboard scribbled on by a crayon, and if he doesn't accept it immediately he'll lose his vow by making a forgery check. And patience mantle is just lame. Grey guard seems like an answer to the latter, though I'm not entirely sure how much of that flavor you want... and you're LN, which normally isn't allowed for the paladin variant.
Basically
Alliance: Just badly converted. I don't see how the atonement fits with the ability.
Conviction: Meh, there are better ways of overcoming DR.
Diligence: Subpar abilities. Vague requirements. Pass. I mean, I guess detect evil is OK, but given how much alignment concealing magic is going around, and the fact that you're supposed to be serving law, not good, I'm not sure exactly how much mileage you'd be getting out of it.
Honesty: See previous statement regarding taking counterfeit bills. Abilities aren't particularly useful.
Honor: Cha to attack rolls is nice, and there isn't anything about not being able to GIVE flanking, so this is actually great for fueling power attack.
Justice: Double smite damage. But your DM can easily screw you over with dumb laws. Depends on the DM.
Patience: Normally, mostly worthless. For you, entirely worthless.
Perseverance: Decent. Not spectacular, but decent.
Piety: Given that your party will be sleeping eight hours a day, the five hours of prayer isn't that much of a hindrance. Since you're your own ally, getting either a quarter of your level or your cha mod to saves is excellent. Oh, and you extend your save bonus to the party. A++++, would take again.
Zeal: Combine with justice. Duels can be to first blood, so atonement is easy peasy.
I doubt you'll find more than three or four mantles that you actually want, and there are other ways to get smiting progression, so a good breakpoint would be seventh or tenth level. Divine Champion using the Transformation Domain would be an excellent choice if you break at tenth level - you get shapechange 1/day at level 19, which would let you keep up with krazy kasters.
Third: Heirloomwise, have you looked at founding a Weapon of Legacy?