The canister filter generally won't filter out the fertilizers you dose the tank with depending on the filtration media you use. This is a very debated topic actually, because many people believe activated charcoal/carbon will filter out chelated iron and other minerals/nutrients that plants need while many people believe it won't. I personally don't use activated carbon in my tanks and I'm pretty sure the majority of planted tank keepers don't either but you will find many that do. A pressurized CO2 kit is definitely not essential for all planted set-ups but is highly recommended and can be essential for some plants. Many people use sugar/yeast CO2 set-ups but on a 55 gallon tank it's pushing it. Many people dose flourish excel as a source of carbon for their plants but it isn't as effective as CO2. As for the initial cost of a pressurized system, that varies widely as well. I bought my very basic set-up for $115 used which included a 5lb aluminum cylinder, regulator, and a needle valve. You can add ph controllers/moniters and a solenoid to the system to automate it, but that can cost from an additional $100 to who knows.
