Okay, so buy the biggest size tank you can since the extra volume will help with water parameters and in some cases can let you fit another fish bioload in there as long as they aren't cramped in the main tank. I assume you don't want to break down your tank to drill it so buy a HOB overflow box. Since your tank is so small you can rearrange the rockwork so that you can't see the part thats in the tank. If you know how to plumb PVC the rest is easy. Plan out how many elbows you will need and what length of pipe you will need, make sure the thickness matches that of the overflow. Now you have something that drains water into the sump. If you want a refugium (I would reccomend it, can help with nitrates and phosphates as well as sustain pod populations for your fish) then try and have a glassmaker cut baffles to size. You can silicone them in using any 100% silicone such as GE All Purpose 100% Silicone. Just make sure there are no anti mold chemicals. If you don't want to pay extra for glass then buy some egg crate, cut it to size and silicone that in. Buy a return pump that wont be blowing everything around in your main tank but remember if it says it does 500GPH it will be far less because it's pumping upwards. A pump with a dial is best, so if you want a little more or a little less you can fix it. Put the return in on the opposite side of where the PVC comes in and then either hard plumb with PVC or use some flexible tubing. You can heat the tubing up in boiling water (only put it in a few seconds so it doesn't melt) and then it will fit around the nozzle of the pump. Boom, you're done! Just make sure the refugium is the first thing the water hits, then the section with the skimmer and heater, then the return pump. You can also use a filter sock right under where the water enters the sump to catch any debris, but the overflow boxes pre filter should do that as well.
