Depends what you want to keep. That's effectively 20 gallons US for american readers. The juwel internal filter is hopeless, just run it as a power head, and get another one, or a fluval 4 internal filter to keep carbon in. The waterpumps in juwels aren't very strong, and I think the one in the smaller rekords is only 200 lph. It's up to you if you bother keeping the internal filter box in there, with the heater inside, or if you just rip it out - I can't think of much to do with it immediately. I would also move the heater , at leastto another position in the filter box as I found in my bigger rekord (160) that in it's default position in the filter it wouldn't keep the temp steady. The light is a single fluorescent (normal output) which is likely too weak for most 'reef' creatures, but there retrofits available, including T5 adaption kits. A hang on skimmer would be nice but will involve carving the hood.
I considered attacking a juwel this way, but never did it. Whenever I see reports of people just using the juwels as is, they always seem to be having hassles as the filtration and water movement are just not good enough.