There will be no problems converting the tank from SW to FW. Just make sure it's rinsed well. If you used any type of copper treatment, then some of it may have retained in between the silicone, so could be very harmful if you plan to keep FW inverts, such as shrimp, snails, etc.
The general guideling for stocking an aquarium is 1" per 1 gallon of water. Please keep in mind this is only a general guideline, not an exact science. This does not account for the larger bodied fish, swimming levels, aggressiveness, etc. Please do a search on the forums here, since there have been resent discussions on the matter.
IMO, a planted tank is a nice tank. It just makes it look more natural and can be benificial to fish. Seachem Flourite, Ecco complete, or ADA Aquasoil appear to be the most commonly used susbtrate for planted tanks, besides regular fine gravel or sand.
If you want a quiet filter, I would not use a sump with an overflow. A canister filter would be the best suggestion, since you can inject CO2 (if you desire) without worrying about outgassing it. Besides, you cannot really find an overflow that will support a 20 gallon tank.
The lighting with the Eclipse is not sufficient for a plants that require high lighting, but they can be replaced (DIY). I remember reading some info on this on the web. If I find it, I will post it. If you plan to keep the same lighting, then you can go with a low tech tank (No CO2 and lights under 2 watts per gallon). You can also have an open top tank, which allows you to use any type of light fixture than can be supported by the tank frame. this will also allow you to use additional tubes, hoses, etc. from canister filters, CO2, etc.
Suggested filters would be either Rena FilStar XP1 ($60 USD) or the Eheim Ecco 2232 ($70 USD). IMHO, the Eheim Ecco would be better price, since it already comes with the media, so you can use it immediately out of the box.
Substrate will vary depending on what you want to use. You can also use regular pool filter sand, play sand, or sand blasting sand (silica sand), which can be cheaply purchased at your local home improvement store.
I have 2 succesfull low tech planted tanks (both 20 gallon longs), with black sand (similar in size as pool filter sand), lighting at about 1wpg, no fertilizers. Low tech planted tanks are low cost to maintain and operate, since they use standard lighting, little or no ferts, and trimming/clipping is kept at a minimum, due to slow growth of plants. High tech planted tanks require high lights (above 2wpg), CO2, and additional ferts. You will get fast growth with plants, so trimming/clipping will be more frequent. You have more plant selection in a high tech tank, compared to low tech.