What is this already?

Hi and welcome! I think it depends on the size of the skimmer, is it big enough for your tank, and how old it is, I'd probably google the type of skimmer to see what it rates as and sells for as new. What other equipment are you going to use, like phosphate or calcium reactors, powerheads for flow, type of lights you're using, what you plan to keep in the tank, which looks nice, by the way!

 

we were posting at the same time. Cost of a sump depends on a few factors, there are lots of ideas here in the forums for you to consider if you decide on a sump in the future. The main thing is how you're going to get water from your 45 down to a sump and back up, for instance. I suspect the tank isn't drilled to have an internal overflow and return pump system, but lots of people have external overflow pipes and return pumps to a sump, so again, lots of options, nothing critical that you need to rush out and do. Going slowly is so important.