What specifically leads you to believe that your wheels need replacing?

If you do need to replace them, it's often cheaper to buy pre-built wheels than to buy parts to build your own. Buying complete wheels also prevents things like buying the wrong length spokes, or the need for a truing stand, not to mention the time/effort/ability to build wheels.

If you choose to buy something, regardless of what it is, you'll want to know what size/type parts you're replacing. We know you've got a 700c rim and that you run 35's, so we've got the info we need for the rim. A Trek 720 probably has 135 mm rear dropout spacing, but you'll want to confirm that by measuring. You've got 7 cogs on the rear, but are you sure it's a cassette? There's a good possibility it's a freewheel.