Batteries are a different story. The new tanks come with Li-Ion batteries and need a lipo style charger, or the wall charger that comes with the tank. Most guys quickly upgrade to a decent charger and better batteries. I use both Nimh and lipo but a lot of guys are a bit shy about lipos. For what you're doing, you'll want a decent charger, which can be had at Hobby king for about $20, and probably two spare batteries. On a decent charger a 2000 mah nimh will come to full charge in about 30 to 45 minutes, and you'll get about a half hour run time with an all plastic tank. Your local hobby shop should have batteries, or you can get them online. At my LHS they have the Onyx 3000 mah nimh for about $22 each, but they can be had cheaper online. For now don't look for anything over 3000 mah as the price increases drastically above that. You can get a 3000 nimh for about $22, but a 5000 mah nimh will cost more than $50. Doesn't make sense to me either, but that's the way it is. At first you'll be able to charge the batteries fully and play for about 30 minutes, but once you add a good charger and a couple extra batteries you'll be able to play for much longer.
Metal parts are easily obtainable from a wide variety of sources, like Imex/Taigen in Florida or Matomart in Canada. Of course, shipping from US vendors will be cheaper. Installation is not difficult and there are lots of threads and videos to guide you.
6mm airsoft BBs can be had at Walmart, but you want to keep an eye on quality. The BBs that come with the tanks are junk and I have thousands of them that I'll never use. When shopping for BBs the most important word to look for is "seamless". I usually get mine online, and I always look for seamless double polished in a .12 or .15 gram weight. The .2 are good also, but once you start getting up to .25 they get a bit heavy for tank cannons. They're the same airsoft BBs they use in the rifle and pistols you can buy.
As far as I know the cheapest Jagdtiger you'll find (and you'll have to wait for another sale) is $399 US. I don't think that comes in an all plastic version, and as far as I know Torro is the only company making an RC RTR Jagdtiger.