If full reef is likely later, it would be easier to convert over later if you go FOWLR now
You could set-up a bio-mechanical set-up "on the cheap" with what you have, plus a Protein skimmer unless you really enjoy water changes and have a lot of cash to burn on salt...
Basically, once the FW tank is ripped down, you clean everything thoroughly, then fill it up again with salt water, before starting a fish less cycle on it after adding in all the equipment. When cycled, you start adding your fish
This is known as a Bio-Mechanical set-up, and they tend to be high maintenance. You would run it with 25% weekly water changes if you skim, probably nearer 50% weekly will be needed without a skimmer. This system is not suited to most corals though, and many fish types will struggle in a Bio-mech system and it's prevailing high Nitrate and Phosphate problems associated with running a tank without Live Rock to remove them.
The Better way is to add about 10KG of Live Rock for that tank size, and a pair of Korailia 1 or 2 power heads, or my preference a pair of Tunze nanostreame 2065's. You current heater will work with this, and your current lighting will keep you going until the reef conversion, and then you may require a lighting upgrade depending on the corals you wish to keep...
HTH
Rabbut