Hi Singman
I see George (Griss) already welcomed you so I'll skip that formality.
Any size tank will benefit from adding a sump as it adds to the overall water volume and provides enhanced circulation. That said, tanks 55 and below often do well with a hang-on and it does reduce costs. Both the CPR backpack and Remora are very popular hang-ons. The two skimmers you mentioned in you other post are both popular. It is very hard to recommend a certain skimmer simply because there are so many and I've only had experience with a couple. You probably want to run an RC search with any brand you choose to see what comments it has drawn.