Hello and Welcome!
You have some really great advice about how to change up your system to get read for a reef. Lights, flow, fixing up the sump and more rocks are your major items. I would also looking into a RO/DI filter. Tap water can cause huge issue in saltwater tanks, fueling ugly algae growth and in houses that have copper pipes, kill inverts.
However, high nitrates (unless the are scary high, like 100+ ppm) isnt dead to most fish. You probably lost most of your fish through improper acclimation or adding too many too fast. You should only add one fish every 3 weeks to allow for the bacteria to grow to handle the new waste load before throwing more poop in to the mix. If you dont do this you can cause the tank to go through a mini cycle, which is deadly to fish and inverts.
Also while you are re-doing your tank I would get rid of any remaining fish to make sure the tank cycle properly. Plus it looks like you have a damsel in there, which are rather poor choices for any tank. They are so aggressive that they are known for bullying fish much larger than they are to death.
Research every fish or coral before you buy!!!! There are many fish out there that have no business being in our tank, or your particular tank. That doesnt stop LFS employees from selling them to you, most because they dont know any better and its their job to sell stuff. The same goes for equipment.
The last piece of advice I have for you is, go slowly. Nothing good happens fast in our tanks.
