With the light, just check to see if you can find enough positive reviews online about them and consider any problems mentioned in the reviews. Questions to ask before buying is how readily available are parts and what sort of costs are involved if the unit needs repairing/maintenance? How do I change an LED bulb if one blows? Can I get it repaired locally? And I mean after warranty, not during.
With the skimmer, it sounds pretty noisy to me and I'd consider it a cheapy. That noise will get to you after a while, especially if the tank is in a bedroom. Having it enclosed in a cupboard may help noise-wise.
I think 20kgs of live rock is fine in that size tank. Keep your aquascape as open as you can so that flow can get around and through the rocks.