You're in luck, the hardware you have is all very well suited for keeping a planted tank. Honestly, I wouldn't change much of anything except to make the lighting more suitable for plants. You're not going to need to run most of the external reactors, like phosban and such...and definitely not the skimmer. The good news is they make great CO2 reactors.
Lots of people debate the choice of canister vs. sump and in the end it really comes down to personal preference. I personally dislike the maintenance on canisters and, since your tank's already set up for a sump, I'd be inclined to keep it as-is. You will want to find a way to get some mechanical filtration going in there. That can easily be accomplished by making a rack of some sort to hold landscaping lava rock you can find a home improvement stores or garden centers. Some PVC and eggcrate will accomplish this nicely. Cut out the first glass pane you've got and move it up so there's a couple inches of room for water to flow underneath it and up through the lava rock. Add a sponge along the intake side of the baffle and voila, good mechanical and biological filtration.
Another mod you might want to consider is getting a piece of glass cut to cover the whole sump and use flexible tubing leading to a bulkhead through the top pane for your downdraft instead of the hard pipe. This way you'll be able to seal the sump to reduce gas loss. You might need to put a stub of PVC on the other side of the bulkhead depending on water level. Just like a reef, you want the outlet under the water's surface (but not too far under). The threaded portion of the bulkhead might be enough though. I had a system similar to yours (90g) with a homemade 20g sump and it was quite efficient CO2-wise.
How many gph is your pump putting out right now? If you modded the impeller to be a needle-wheel, what would the output be? One thing you can do, and I'd suggest doing, is T-ing off the return line and using your pump to power a CO2 reactor. If you have it feed directly into the sump you can blow as much water through it as you want to reduce flow into the tank to what you want it to be. Think closed loop with the sump as the tank. If it were my system, I'd run that line to output near the tank downdraft. That way you don't have to buy a new pump and will get good reduction in microbubbles from the CO2.
Now, let's talk substrate. If you're going to go the soil route, I'd mix 50/50 turface and whatever soil mix you choose. I found in my research that adding turface encourages root growth by keeping the soil from getting too compacted and providing surface area for fine roots to attach to. You can go with whatever cap you want, but I would suggest something that's not too fine. Grain size smaller than 2mm really is too fine. You want there to be some space in the cap for oxygenated water to diffuse into it. This will create an area in the cap for nasty reduced chemicals to oxidize to insoluble or non-toxic forms.
ABSOLUTELY USE CO2 FROM THE START.
Plant heavily too. By heavily I mean fill as much of that tank with plants as you can. If it's a thick jungle of stems that you can barely see through at first, excellent. Anticipate spending $200 or more filling your tank if you go through regular retail or hobby channels. If you still have access to wholesale you can do it for about $100. If there's a local hobby club nearby or you an find someone online wanting to send you a massive load of clippings you've hit the jackpot.
That's probably enough for now. I'm happy to answer any other questions you've got.
Welcome to TPT and the aquatic gardening hobby!