I'd suggest trying something cheap to get some experience and better learn what you specifically need. In the $25 range, Ebay has a lot of 13-28 7-speed freewheels by Shimano or Falcon, which should just screw right on to your hub. Guessing that what you have there is a 13-21 or 23, a 13-28 would drop your lowest gear ratio by around 30%, which is a huge difference. It would be enough to let you see if you like the new riding experience, and to assess the need for a triple. Plus, with a racing chainset like that, it's very likely your Campy derailleur can actually handle that new freewheel with no problem.

If you buy it at a shop and have them install it, they should also address any issues with chain length as well. It could need to be adjusted. It would cost more at a local shop, but you should come out good to go.

On that chainset you probably can't get a smaller chainwheel. Most that look like that had a bolt circle diameter of 144 mm, and 40 teeth is about the smallest that can be fit in that case. To avoid the much bigger cost of changing the chainset, do the freewheel first to see if it gives you everything you need.