The scariest part to me in the beginning was lights.
A 20 gallon reef tank will be much more difficult than a 75 gallon. Evaporation happens, water goes, salt stays, all the sudden your water is much saltier than before but you haven't put salt in it. Science. The bigger the tank , the easier, until you get into the monster tank status then it's a different conversation. Don't limit yourself on size, dream big, because the bigger the tank, the more critters you can keep.
Lights have 3 categories - metal halide (MH), flourescent (T5), LED . Each of them are great, and each person will tell you why one is better than the other two. If you're looking to spend $200, try to go the used route. You can get a couple LED bars for that price used, but I am personally a fan of the T5. I spent $600 on the LED bars, and my corals did not die. I supplemented with four T5 bulbs, and my corals instantly took off. There is more to it than that, but I'll never have a reef without some T5 bulbs providing a large portion of the light.
Everything you do, do it slowly. Add fish slowly. Add coral slowly. If everything has been okay for a month, add another fish, or add another coral. It's hard to do it this way, but if you don't then you run the risk of everything dying anyway. If you are doing something wrong, you won't know it until it is most likely too late and the consequence is right in front of you. Don't hesitate to ask questions, and ignore the dumb mean people, they have no other place to belittle people so they unfortunately decide to use this forum. "Keyboard warriors"
Quarantining (QT) your fish is a great idea. Some people do - some people don't. Make a decision about it before you add your first fish, or it is already toolate.
Good luck, best of luck, the freshwater tank is beautiful !