Making the leap from freswater to Marine

Howdy all. I've been lurking a bit, and reading posts, and my desire for a
marine aquarium is growing ever stronger. I have kept tropical aquariums in
the past, and am planning on creating a "river tank" set up that is half
land and half water to house some fish and amphibians. However, I was in
the local pet store the other day and was, as usual, drawn to the beautiful
marine life for sale. I have two empty aquariums that are currently
suitable for a full aquatic set-up. One is a 20 gallon high (I'm thinking I
don't want to deal with water quality in a marine tank that small) and the
other is a 40 gallon long. Is the 40g tank big enough for a beginner
without being too big? All I have is the tank, what sort of filtration and
lighting do I need? I'm not trying for a reef tank (although I will
someday) since I can't afford live rock, or the lighting coral requires.
Can I set the tank up similarly to a freshwater, but with salt-water and
cycle it similarly? I've heard blue damsels are good fish for cycling the
tank, is this true? Is there a good marine life book for beginners?

Lastly, the two animals that really caught my eye in the shop are a black
and white polkadotted eel (I think the tag said spaghettie eel) and a lion
fish. I'm assuming neither of these animals are suitable for beginners.
(isn't the lion fish poisonous?) Do these animals need a reef tank, or will
they fare o.k. with non-living decorations? What are some good marine fish
for a first tank? Can animals like starfish, octopi, anenomes etc. survive
in a marine tank that is not a reef tank? Is there a faq for this group?
Are marine tanks significantly more difficult than Freshwater tanks?

Thanks for any replies!!

Emily