The holy avenger has a + 7 equivalent in the hands of a paladin. (+5 and holy which is + 2).
You cannot add enhancement bonuses to a holy avenger because it already has +5 for you but you could add +3 worth of the allowed properties.
So, I can add +3 Defending then...that'd be cool.
You state it's a +7 equivalent, but how about the SR and Greater Dispel Magic enhancements?
What are the values of those enhancements?
Are those even enhancements a paladin can add using his divine bond?Whatever the case, the weapon effectively functions as a +7 weapon in the hands of a paladin. It means you can add +3 worth of stuff.
The enhancements I mentioned are already part of the Holy Avenger. So, you're basically saying that those enhancements don't increase it beyond a +7 weapon as it stands in the hands of a Paladin?
If those abilities do not fall under the chart options for +'s of a weapon enhancement then they are part of the extra gold cost of a specific weapon.
So yes you have three open +'s of room on that holy avenger to use divine bond with.
Like Gilfalas said, those are specific weapon properties on a holy avenger rather than "weapon properties" the game term that have associated costs. They don't take up any of your bonus potential.