You do not want to use an undergravel filter. These filters are notorious for "going bad"--that is, getting waste stuck under them and causing ammonia spikes that can kill your fish--and so require very diligent upkeep. Even a well-kept tank with an undergravel filter needs to be broken down once a year so that underneath the filter can be cleaned. It's really not a viable filter.
A Magnum 350 is, from what I have heard, a fairly decent cannister filter. The only thing I'd be concerned is that it might not be enough for a 55 gallon. However, if you are not heavily stocked and clean regularly, it should be alright. If you still want a second filter, though, you could always get a second cannister filter (or even just upgrade to a higher-rated one) or get a HOB
filter to supplement the one you have.
As far as cleaning the film, you can buy magnetic scrubbers than work well on acrylic tanks, or even use a toothbrush to scrub off the difficult stuff. Most film will wipe away fairly easily, though.