Tbh Saltwater fish keeping is not that difficult now. I say this because years ago I personally didn't have the internet to do research and there was very limited information in books, magazine's. Lol I had to buy expensive $80.00 Metal Halide Bulbs to have proper light along with cooling fans and ballist with Atinic flourescent bulbs. Now you have less expensive LED fixtures and better equipment. The only advice I can give is keep doing what your doing research, research, before plunging in.
Lighting
Im not sure how less expensive leds are compared to halides, currently I'm running 2 radion xr30 pros and the lights with mounting kit costs around 2k. Kessils your looking at a price point of about $500 ea if your running just one with a hanging feature and only covers a 24"x24" area. hydras are up there same with orphek being up there. Lighting is the most important equipment in a reef so the majority of the cost of the set up will come from here. Get an elite brand, you can't skimp out if your doing coral its their most important lifeline. If you decide on doing a fowlr tank instead then lighting doesn't matter much you could do a current orbit or something.
Filtration
I would only feel comfortable using a sump for a reef for these following reason. Sump tanks let you extract the waste a lot easier by visually seeing how dirty the filter socks are, you can also run chaeto or another macro algae to eliminate nitrates, and you can use an in sump skimmer to extract waste not to mention hide your other equipment like heaters. In reef tanks its important to keep the nutrients low because small amounts of nitrate will irritate corals which you don't need to really worry about if your doing fish only. Canister filters are normally viewed as nitrate factories and I agree with that saying. They do a great job of collecting waste but not removing it. I would not use your fx4.
Corals
Even easier to care for corals can prove to be challenging due to the water requirements needed for them. The best options for beginners is a softie tank. Soft corals are a lot less maintenance then lps and sps corals because they don't extract calcium and other minerals in the water nearly as fast as stony corals do. Mushrooms, green star polyp, Kenya tree, devils hand, and any sinularia (finger) coral really are the best choices for beginners.
Stock
Mandarin gobies are very challenging, I would hold off but if they are a must for you I've gotten them to eat nutramin ova. They love very small food items and copepod colonies end up getting wiped out very fast it helps to have both when caring for a mandarin goby though. If you add a mandarin keep it with tank mates that aren't aggressive eaters. Cardinals, Clownfish, Pygmy angels, Sixline wrasse among a lot of others are good for smaller set ups and beginners.
Clean Up Crew
Clean up crews help maintain the algae in the tank but water changes are still needed. Im a firm believer of only using snails because hermit crabs do eat snails when they fall over. I use tiger sand conches and fighting conches which do a great job on cleaning the sand. They eat diatom algae as well which is a huge bonus. Mexican turbo snails do great job eating algae on the glass and in the overflows. Ninja star snails do a great job on the rocks as well as sea urchins if thats a critter your into.
Water changes
Everyone has a different schedule due to stock, lifestyle and how much they feed their fish. I personally do a water change every week of 25%. For me it works well because I have the time on the weekends, I get to replenish the minerals consumed and I like to feed the fish and coral often because I see good results from it. I feel like it builds the fishes immune system up and corals gain size with added feedings along with good light. if you watch bulk reef supply vids they'll tell you its not mandatory to do one more then once a month under some conditions. Dosing elements with a doser will eliminate the need to replenish through water changes. Low stock also reduces the need along with low feedings. Bulk reef supply is coming out with a video series of an ultra low maintenance build. Please give these videos a watch, they're on youtube and their site.
I think I covered everything you asked in detail
