upgrading

cheapest way would be to buy a 110 gallons second hand marine aquarium set it up transfer the fish and either sell your 55 gallon or keep it and use as a sump or something. Filtration, the best filtration is LR, buy about 1/3's worth of LR to fit in your tank, for mechanical filtration you could use the 55 gallon as a sump. Hang on filters are basically the same as cannisters just no were near as efficient and they are harder to clean. if i was you i would use the 55 gal as a sump, and buy 2 emporer 400 to circulate and another pump to pump the water back from the sump into the Aquarium. also are you making it reef or fish only. And if you stock it lightly you could get away with using a good protein skimmer and thats all, but i would prefer to have a cannister filter to remove all the crap out. Just remember if you get a cannister or hang on, regular maintanance is essential to keep nitrates at 0.