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Eheims are generally top of the line filters. I have seen some Eheim HOB filters, but have not read any comments about them. Aquaclears are very popular with hobbyists here in the states, and they are less expensive. ATM is right on about how many gph you need. Don't go by what the box says when it tells you it's rated for a 15 gallon tank. Look at the gph rating instead.
CO2 is only necessary if you are really into plants and have high lighting. Although some of the most beautiful tanks you see online are run with CO2 beside the added expense, they also run the risk of killing all their fish if the CO2 system malfunctions. The CO2 affects the amount of oxygen in the tank and too much can suffocate the fish. My recommendation is if you are new to plants, and want to start slow and easy, go for the low tech tank which can be quite beautiful. There are lots of plants available out there that don't require special lighting or CO2, and look great in your tank. Here's a great article about starting a planted tank that will help you out a lot:
http://www.plantgeek.net/article_viewer.php?id=17