The Fluval 405 is a replacement for the 404, which most people have mixed oppinions about. Most of the time, the o-ring that seals the canister fails and starts to leak. A leak in any canister would be a disaster if not caught, you could lose many, many gallons of water onto the floor, possibly damaging the stand, flood, carpet, etc. In worse cases, fish may die from lack of water and/or filtration.
The 405 goes for $200 at Petland, but should be cheaper elsewhere, like on ebay or other large retailers. I would imagine that you can purcahse a new one for $150, or a used one at $75-100.
The "system" does use filters. There's room for 4 foam pads, and 4 compartments for filter media. You can pick and choose the type of media you wish to use. Most would use BioMax, charcoal, Pre-Filter, and BioZorb. Each has it's own abilities and purpose, you should look into those more yourself to see what type of fish you are keeping.
The 405 is rated for 100 gallons, would be sufficient for a 55. You could also get away with a 305, but would require more frequent maintenance.