Learning to weld with a non-aircraft project

Eventually I plan to build a tube-n-fabric aircraft, but now isn't the time.
However, I would like to start developing the skills that I will need.

I've been looking at kids playsets for our yard. Most of them are made from
redwood, and are quite expensive. Several thousand dollars for the more
complex ones. Pressure treated lumber is much cheaper, but I don't want my
kids crawling on it.

Today I got an idea. I could build a frame out of steel tubing, get it
powdercoated, and then deck it with the composite synthetic decking that is
becoming popular for backyard decks. Such a structure would be strong,
durable, impervious to the elements, and would give me an excuse to learn to
weld.

What are the minimum set of tools I'd need to buy to complete the project?
I know I'd need an OA torch. What tools would I need for the cutting,
shaping, and prepping of the steel tubing? Any other suggestions on
construction and materials, especially steel type?