Fresh to salt. Easy. The big differences are salt water tanks have more lighting and more water movement. You are going from lake fish and plants to reef fish and corals. Use the canisters. Clean them often. They dont produce nitrate (nitrate factories). They do filter your water and a reef tank needs to be a lot cleaner than a lake fish tank. You will want to change your water more often. Maybe 10% a week and keep the tanks salinity stable. (Top off every day with fresh rodi water.)
For a 90 you could get the 2 Grye's or 2 Jebao powerheads
https://www.bulkreefsupply.com/3k-gyre-generation-powerhead-icecap.html
https://aquarium.bulkreefsupply.com/reefing/Jebao-Powerhead
You want random flow, so point them opposite each other or bounce them off the glass
You can have too much flow depending on what you want to keep. Some fish like fast current some like the lagoon. Same with corals. For starter I would stick with a few easy to keep corals. Medium flow. The fish, you have experience so you know diseases, compatibility and stuff. Live rock will handle most of your ammonia to nitrate conversion. The way you decide to export those nitrates can be a skimmer, vacuuming the sand bed, water changes, live macro algae. ect..
The lights dont have to be fancy things. The Chineese black box lights will work just fine if you dont get the super cheap ones. Something like,Ocean Revive T247-B SBReefBright, Mars Aqua, Viparspectra's will be just fine to start out with.
You will do just fine, so have at it.
Once you "get the hang of it" then get the big tank, sump and all the goodies