I dont know much about this type of tank, but a general idea is that smaller tanks can get away with being skimmerless as long as you keep up with your weekly water changes. I didn't see much change on my 36 gallon from my skimmer in all honesty. But I have been told that these smaller all in one setups can do fine without one. If later on you want to add one, then I'd evaluate it at that time.
If you are wanting to have a reef, first evaluate your light in terms of what kind of coral you want to keep in it. Soft coral don't need much light, but LPS
start becoming demanding of more intense light and SPS
and anemone require high intensity lighting. Stock tank lighting usually just doesn't cut it.