Welcome to the forum!
You're correct that the protein skimmer doesn't work with fresh water so it has to go.
You could continue to use the sump for filtration as they are excellent even though fish keepers with most large fresh water tanks would opt for a good canister filter.
But since you already have the space and the sump, I'd sure use it and I'd set it up pretty much the way you had it for SW with filtration media.
(Especially) If you had corals, you might have been using RO, DI, or RO/DI water to mix with sea salt? You won't necessarily need this with FW, although you can use it if you treat for minerals, trace elements and pH. However, most use treated (to remove chlorine) tap water. I suggest you test your tap water using the API Freshwater Master kit. I have well water with high nitrates and although nitrate levels are perhaps less serious in FW than SW, they are still a concern.
On the subject of water, you'll want to do weekly water changes to keep the water fresh. This can be 20-50%.
I presume you know all about the N2 cycle since you had a SW tank, you probably had live rock and live sand? You might wish to use one of the bacteria starters in your new FW setup (Tetra SafeStart, Seachem Stability, API QuickStart to name a few).
For substrate, choose a fine gravel or better yet sand. Many report success with play sand. I am using pool filter sand as it is typically slightly larger grain that will not compact. I feel sand is better because fish/plant waste and uneaten food can't get down under quickly like it can with gravel. This means no uneaten food and mulm slowly decays on top (or it can easily be siphoned off) just as in nature.
Consider having lots of plants. Plants (both rooted and floating) are not only aesthetically natural and pleasing, but help filter the water. You may need to examine lighting and use some type of fertilizer (root tabs and or a liquid like Seachem Flourish).
Decide on the type of tank you want and choose the right fish. I like community fish, while many prefer semi-aggressive or aggressive. There is a lot of good information in the 'Tropical Fish Profile' section of the forum (top of page).
Only add a few fish at a time and even though you have a large tank, be careful not to over crowd. I know a few fish seem lost in a large tank, but growing stock slowly and ensuring they have ample space pays huge benefits. Bear in mind that many shoaling fish are most happy in larger groups. A dozen or more neon tetras or zebra danios are a sight to see in a large tank.
Another consideration is fish size. You have room in a 200g tank for fish that grow large, but remember that big fish see little fish as food!
Okay...best of luck and keep us posted.