The Westside C-16s, like most brass locomotives, were made in batches many years ago, so you need to find them on the used market. eBay is probably the best source, but there are dealers with on-line inventory and other auctions sites.

Soldering brass pieces together is very different from soldering wires. With wires, you use a rosin-core solder and don't care about a big solder blob or accidentally unsoldering another part nearby. That doesn't mean it's something that you can't learn. I was about your age when I first modified a brass locomotive, and was able to work it out with advice from friends that I met through the hobby and magazine articles. I did a quick search on YouTube and there are quite a few instructional videos, so that would be a good starting point. For HOn3, you may be able to get away with a soldering iron that was intended for electronics, but as the parts get bigger you need more heat storage (bigger body and tip) or faster delivery (torch) than those little irons have. I haven't done much soldering in a long time, but I generally used a thermostatically-controlled electronics iron for smaller pieces, and a big iron, resistance soldering unit, or small jeweler's oxy-acetylene torch for larger parts (including soldering small parts to large ones). The keys are to have clean surfaces, flux where the solder needs to go, the right solder alloy, getting enough heat onto both pieces being soldered so they can melt the solder (the parts need to be hot enough for the solder to metallurgical bond), and not too much heat anywhere else. For solder, I generally use Sn63 for most things, which is the eutectic ratio for lead-tin (which means it goes from solid to liquid without a paste stage). If I'm concerned about a part coming loose while heating nearby, I'll use silver bearing solder from the hardware store. Some people get carried away with several different alloys with different melting temperatures. To control heat, you can buy commercial heat sinks to keep previously-soldered parts from getting too hot, or use wet tissue. I suggest practicing on something cheap, such as brass parts from the hardware store or a cheap brass kit or model. You could try to pick up something like this FED mogul as an experimentation project if you can get it cheap enough.