Well to start off what profession are you getting weapons for?
Also what kind of build are you using?
Some general tips for melee/physical damage dealers:
Sundering is one of the worst possible mods you can get. You'll do more overall and consistent damage with a vampiric weapon. 20% armor penetration will only do a couple of extra damage and as I said sundering is inconsistent with only 20% chance with each attack.
It's always wise to upgrade your weapon based on your build to optimize its effectiveness. For instance you wouldn't put a vampiric or sundering mod on a bow a cripshot ranger is using. You would put a crippling bow string on to lengthen your foes crippled time since thats what your build is going for. So be mindful of what your build is trying to do when choosing weapon modifiers.
Now as to how to build it:
If you're going for a perfect weapon make sure to buy/find one with a perfect damage modifier.
Example a weapon whose requirement you can meet that has an inherent mod such as the popular damage+15% while health is above 50%(aka 15^50). There are others such as damage +15% while in a stance or damage +15% while enchanted, etc.
Now aside from inherent damage mods there are a few items that come with built in mods that cannot be changed out much like damage modifiers. These weapons include the Fiery Gladius, Fiery Dragon Sword, Icy Dragon Sword, Icy blade Axe, Fiery blade Axe, etc. These weapons have a built in damage type modifier IE Fiery blade axe deals fire damage but that mod cannot be changed, it is permanent whereas most otehr weapons don't have this restriction.
Weapons can have a prefix and suffix mod. AKA: "Prefix Fellblade of Suffix".
A weapon may only have one prefix and one suffix attached to it at a time. Adding another will overwrite the old. So if you were to upgrade an "Ebon Gladius of Swordsmanship" with a Vampiric Sword hilt, it would replace the ebon prefix and become "Vampiric Gladius of Swordsmanship".
To upgrade simply double click the upgrade component and then click on the weapon you want to apply it to. Once you've used an upgrade it is bound to the weapon it is on. You may later salvage it but you will have a 50% chance of destroying the weapon in the process. This percentage decreases by 2% with every rank in the Wisdom title.
Hope this helps.