Well I'll start off by saying that the iron test kits are mediocre at best. Most people don't bother with them. Iron is present in trace ferts and can be supplemented with Flourish Iron or similar, should you notice a deficiency.
Ammonia test kits are pretty much a standard item so I'd recommened you get one of those.
Since you are going to be setting up a CO2 system you will absolutely need to know your KH (carbonate hardness). The Aquarium Pharmaceuticals test kit comes with a test for both KH and GH which is handy. If your KH is below 3dKH you'll need to buffer your water to prevent a pH crash when injecting CO2. Some aragonite in your filter is a good long term solution to this problem, which I have myself. Baking soda can be used as a quick fix.
By using your pH and KH you can also determine the CO2 content of your tank water. I'm not sure I'd trust a CO2 test kit unless it was a probe of some kind.
Hope this helps. By the way, what tanks size and lighting have you got there?