Definitely do some research on the anemone before getting one. They're fairly demanding animals to keep healthy if you're new to the hobby. They need pretty good water quality. Plus, as strange as this sounds to someone new to the hobby, clownfish are by no means guaranteed to host an anemone. There are lots and lots of tank owners who can't seem to get their clownfish to pay any attention to the anemone in the tank. An acropora colony with a crab would be a good example also, but acropora are some of the hardest photosynthetic corals to keep in the hobby. Again, definitely do the research on those before attempting this pairing. Some other colonies, like Pocillopora (which are a bit easier to keep) can also host crabs.
The cleaner shrimp is a very good option. They're relatively hardy and provide a lot of personality in a tank. Mine would actually hop on my hand and start cleaning it whenever I put my hand in the tank.
Almost any photosynthetic coral is a good example of symbiosis, although it's not as flashy or obvious as an anemone and clownfish. Every photosynthetic coral contains an algae (technically, they're dinoflagellates) called zooxanthellae. These algae live inside the coral's flesh, use light to make food, and in return for the protection, they give the coral some of the food. As a result, the coral, which is a living animal, usually needs no supplemental feeding. Most corals are supported almost entirely by the food produced by the algae in their flesh. The zooxanthellae are actually what give corals their color as well.
As for cheap equipment, Craigslist is definitely your friend. It's possible to set up a smaller tank for a few hundred bucks, but it's definitely not a cheap hobby, especially if you want to support animals like anemones and small polyped corals.
Please feel free to ask if you have any other questions. This is a great community with a lot of very knowledgeable members.
