As I was deciding what I would use to build K-9's chassis and body framework, the immediate thought I had was to use an Erector set. I never had one while I was young as I preferred LEGOs as a kid. However, it seemed to me to provide ease of construction, versatility of parts, and yet still be lightweight so as not to bog down the servos with weight.
Since I never had a set before, I wasn't sure what set to start with. That meant turning to eBay to see what kits were available and hopefully at a great price. As tends to happen, my fact-finding mission quickly became a chance to place a bid. The kit I thought was the best value and provide the most parts was a set that was called the Multimodels 25 Super Construction set. The same set was released for the 100th anniversary. It has been discontinued but is pretty easy to find on eBay. Prices tend to max at around $90 plus shipping, but I found a set that was asking for a best offer. I offered a decent price and the seller countered, and I accepted the offer. I bought it for $37 + $6.95 shipping. The great news was that by paying with PayPal I had some money sitting there. So, in the end, it only cost about $25 out of pocket. Score!
The set arrived yesterday and it included everything but the instruction manual. A quick Googling tracked that down too. Here's the link if you are curious: http://cdn.meccano.com/notice/837080.pdf
So, in order to get familiar with the parts and tools, I plan to build one of the models -- a car -- and maybe the sweet catapult (because how cool is that?!) but then I will draft up a frame and chassis for K-9.