Not sure bout Juwel tank, but here's my 2 cents.

SKIMMER
Get any venturi skimmer. Most in the market are venturi type anyway. Skimmer is one of the most important equiptment in a marine tank, so get the BIGGEST that you can fit in your tank. There are some hang-on-back skimmer, and some small ones that fit inside the main tank, but those are small ... I guess better than not having one!!!

LIGHTS
Not a big issue if you do not have corals or algea / plants.

SIZE
4 ft is the recommend MINIMUM size for beginners. Smaller tanks are for EXPERTS. Reason is that the water condition changes very easily in smaller tanks. This stresses the fishes easily, and sickness and death comes quickly. I recommend getting some "bullet-proof" fishes like the clown fish and demsals for smaller tanks.