The 55l aquanano would be very easy and could be quite a cheap way of getting a marine tank. The basic tank with the rear filter is very similar to the plug and play marine tanks such as the orcas etc when I first got into marine I found I got caught in a trap of you must have this you must have that, marine imo is no more difficult that tropicals with a bit of research and low stocking it really is a case of keep it simple, as a rough guide your 55l could easily house a pair of clowns a pair of cleaner shrimp 10 snails and 5 hermits,
Corals are where things can get complicated but if you went for soft corals such as xenia, zoas, mushrooms etc basic t5 lighting would be more than enough
Budget wish, with a bit of ebay magic to get your tank up and running with live rock, extra powerhead optional skimmer etc £200 is more than realistic
Hth
Mark