As you have probably figured out marine is more expensive. Do a little searching on the saltwater forum, I don't have a marine tank (yet!), but it looks like to have a complete marine reef setup you would have to purchase a skimmer, live rock, metal halide light fitting, wave maker (fancy pump for getting flow in tank), pump from sump, water testing gear, plus all the livestock. This can get expensive and most of the salties on this site seem to say the same thing "don't buy cheap". Not very many people use canister filters on marine tanks - the live rock takes care of the biological filtration and the skimmer takes care of the rest
You already have all the physical bits required for a Tropical setup - canister filter, heater & lighting. What you would need to add to get a complete setup assuming you would want to go natural is a layer of plant fertilizer, stones/gravel, piece of bog/drift wood, plants, & fish.
You also might need to look at replacing the tubes in you light fitting as it is second hand - recommended to replace each 6 months.
Good luck with either way you go, and welcome.
Cam