Definitely want to upgrade your skimmer if you are going to start keeping sps corals. You may want to look into a refugium as well.
It is possible to go sumpless and run a closed loop and an external skimmer but if you have the space then I would recommend a sump. Having a sump will increase your water volume and increase your water surface area which will increase your evaporation rate to help cool the tank. Not to mention give you a good place to house extra equipment.
If you add some macro algae and create an area for pods to reproduce then you basically have a refugium. A sump can be as simple as a container of water that the overflow from the tank is connected to and a return pump that pumps water back into the tank.
A popular design is to have something like an acrylic sump or glass aquarium that is sectioned off with baffles where the refugium can be contained within a compartment within the sump.
It is possible to go sumpless and run a closed loop and an external skimmer but if you have the space then I would recommend a sump. Having a sump will increase your water volume and increase your water surface area which will increase your evaporation rate to help cool the tank.
Or you could always buy a big oversized chiller to cool the water down and prevent too much evaporation.