You can enchant a shield as weapon, because it is a weapon. You don't need shield spikes.
The weapon enchantment will not stack with the enchantment bonus provided by the bashing enchantment, but once it's higher than +1, it will override. The damage die increase will stay in effect though.
Well, if you are focusing on fighting with a shield, then delaying the weapon enchantments is better. The Shield Master feat will allow you to use the enhancements to AC as an enhancement bonus to attack and damage.
Also, shields made of special materials can bypass DR.
A mithral shield will count as silver for the purposes of bypassing DR.
If you are unsure how to price a shield made of a special material, price it as a weapon.