I'm pretty much still a newbie as well and ended up with a 20 gallon long. I think getting an ro/di filter is a good idea (major algae problems with tap) as well as an auto top off (because some days you feel too lazy to keep up with evaporation). There are cheaper options for both on
amazon. Good lighting is key and there are many good cheaper LED lights now that work fine (I myself have a viparspectra which works well for my simpler corals).
A refractometer is basically a must.
I don't use a sump, but am experimenting with a hob refugium which is about 4 gallons. There are numerous threads/videos about diy hob refugiums or chaeto reactors to control excess nutrients if you're into that instead of dealing with a sump.
Rock can be purchased dry as a cheaper alternative as well. I also use a little hob skimmer which may not be necessary as long as you keep up with water changes and have a low bio-load. A refugium or sump also are not completely necessary with our tank size with regular water changes and there are plenty of examples on youtube (but they do seem to make things easier).
The biggest difference between salt and fresh I've found is basically maintenance. I find myself spending much more time making sure things are ok (checking salinity, emptying my skimmer, making sure the coral are ok, checking the algae, making ro/di water) vs. my freshwater shrimp tank where I just feed and top off every once in a while (I don't even do water changes and they're flourishing). Basically it's just doing a lot of research to see what worked best for others and then applying that to figure out what works for you.
