Hi and welcome to the forums! I hope you enjoy your stay.
If it has an overflow, I highly suggest trying to go ahead and fit in a 55g aquarium under the tank as a sump. It will allow you to have a skimmer that fits inside the sump, as well as enough room to fit in a refugium. You will not regret having the extra sump room. In fact, I believe a 20g tank is a bit too small for a 135g anyways. I've only used 20g tanks up to 90g and had very little room for error. So, I highly recommend a 55g sump.
For a 135g, a simple mag drive 9.5 pump is sufficient. You don't want a crazy amount of flow through your sump to begin with because of your fuge and .... it doesn't really benefit anything to have a high flow rate through the sump. Just enough to let detritus get caught in the mechanical filtration, the skimmer to have some water to clean and the refugium to have enough so that it will grow macroalgaes. A couple of heaters is better than one, just in case one goes out, the other can pick up the slack until you replace it. This helps stabilize the temperature so that your inhabitants won't have any problems.
There is an ultra cheap way, a moderate way and an expensive way to get water flow in a tank. Cheap ways are maxijets with Hydor oscillators. Moderate ways are Koralia powerheads. An expensive way is to go with Tunze or Vortech powerheads. I've done both the cheap and moderate ways throughout my entire hobby experience. I will just now within the next year be venturing out into the more expensive ways for water flow.
I highly recommend and enjoy using Reef Octopus brand skimmers. I have a Diablo 160 and it was so easy to set up and so easy to adjust. Producing skimmate like a champ. SWC skimmers are also good skimmers - any cone skimmer that has a small footprint would probably be your best bet.
Kessils are really good spot lights, but I don't think I would use them on my reef. I'm more of a T5 fan, but it is really up to you when it comes to lighting. Decide what kind of corals you want to keep - that may be the deciding factor on what light you get.
