1. Can I do a FOWLR without specialized lights? (MH/PC etc.)
No special lighting for a FOWLR... If you start adding corals, it's no longer a FOWLR. If you want to start adding corals, buy proper lighting from the start, not just good enough lighting to "get by." The people I know that have bought lighting just to get by were always disappointed and felt the money they spent on lesser lighting was more or less wasted.
2. Is it an absolute must to have RO water for water changes?
The benefits of using RO (or even better RO/DI) greatly outweigh the costs. If you plan on adding invertebrates and/or corals, RO would be a minimum requirement, as corals and inverts will not tolerate some of the chemicals traditionally found in the bulk of tap water supplies.
3. Honestly, how do I do waterchanges for salt water? Like how do I get the salinity of my new water to match the water in the tank?
Using a hydrometer to compare specific gravity levels in the tank and the new water to add back after a water change is fine, but if you feel that you will eventually want increasingly more difficult critters / corals in your tank, a refractometer is highly recommended. Hydrometers may work fine over the long term for some people, but a refractometer is very easily recalibrated to assure accuracy of specific gravity levels. Hydrometers are not so easily recalibrated by the average user.
A skimmer is certainly not a requirement, but can be a very useful tool.
A sump is never a requirement. They are just nice to hide equipment like a skimmer and heaters in, as well as allowing for the growth of macroalgae for nutrient export if a refugium section is incorporated into the sump.
A DIY sump can be far better (and much more affordable) than the commercially available sumps. A basic DIY sump is very easy to build.
