Be careful of what you see or are promised regarding 3D printing. More often than not, it's renderings you are seeing, not actually prints.

Don't get me wrong, there are amazing things that can be done with 3D printers, even affordable units, I use mine almost daily, but you'll need to spend a good amount of cash and a lot of time before you'd get results that would rival a styrene kit. A lot of post printing cleanup, etc. I have friends who use high end machines and they get great results, but the prints also take a long time. I'm talking 20 hours for something the size of a 1/16 figure with good resolution.

In terms of metal, yes, but it's not forged metal and it's not something you can do at home. They also have ceramic, rubber, etc, but those all need to be processed after. It's not a machine dispensing molten steel, think of it more like metal dust in resin. Go on Shapeways and look around, you can get an idea of prices of things.

You could do tracks in PLA or ABS, based on the design, they'd probably hold up as good as nylon ones, but it would take a long time.

One of my future projects will be a Kettenkrad hybrid type of thing, I'll be using the 3D printer a lot, but mainly to make masters to be cast in resin. Casting is still way more economical for production. There's no way to speed up the printing process, you are still dealing with layers and layers of melting material built up. the higher the resolution, the more the layers, the longer it takes.

Perfect example: I always wanted a model of the Discovery from 2001. I scratch built one out of cardstock when I was a kid. There are many free files of them for 3D printing one on your home printer. I downloaded a set and shrunk it so the head was 3 inches across. Cutting it in half (because printers don't do well with unsupported areas) and printing the two pieces of just the head ran about 8 hours. And it still was basically just a Discovery shaped sphere that would require a lot of cleanup and detailing. Moebius just announced they are releasing a Discovery model kit at the end of the year. 500 plus pieces, over 4 feet long and will retail around $150. That's far and beyond a bargain compared to the cost of printing one out and will be light years ahead of it in quality too.

Printers are great, but don't believe any claims of amazing results. We are far from pushing a button and getting a Tamiya kit.