You'll want 535 defense for heroics.
Or 540 defense, if you'll tank raids.

The gear is pretty close to starting heroics.

If you run dailies, you can purchase some Naxx/Ulduar/ToC raid level gear, which will make heroics much easier.
Don't go overboard, as heroics now drop badges which buy gear equivalent to ToC.
If you do choose to upgrade toons, via AH purchases, I'd probably leave it at the tank only.