For the DC, I think 10 + 1/2 Bab + Str Mod would be just fine, but size modifiers should affect the fortitude save, so a large creature would get +1 on his save, and a tiny creature would get -2.

For healing, an instantaneous heal check to stop the bleeding, and either a long term heal check (of the same DC as the Fort save) to repair the whatever, or a restoration (greater?) spell.

A few things you might want to address...
-Are held items dropped when the limb is sundered?
-Are shields still effective after the shield arm is sundered?
-A heal check wouldn't reattach a severed limb, so should a slashing weapon sever the limb or just incapacitate it? Maybe a vorpal weapon severs a limb when used to sunder? Maybe if you do, say, 50% instead of 25% (or a crit and 25%) the limb is severed in addition to incapacitated.
-What are the effects of having a leg sundered?
-Can you sunder a head? If so, what goodness is bestowed?
(it should be harder to sunder a head if you do allow it)
-Does having a sundered arm hamper spellcasters? If so, how?