You can definitely run a sump, but how to configure it will depend a bit on whether you want to run a high tech or low tech tank setup. I'm guessing that you already have pretty powerful lighting for this tank if it was a reef tank in a former light- if you want to make the most out of that, you should look into setting up a high tech tank with a pressurized CO2 setup. A good one would run you about $200 (check out www.GreenLeafAquariums.com
). If you have any actinic bulbs you'll need to replace those as well- bulbs in the 5000-10000 kelvin range typically will support FW plant growth.
If you go with pressurized CO2, you'll want to design the layout of your sump in such a way that the gas won't escape the sump. I strongly encourage you to check out the photo album section and read through some other journals- you'll find tons of great ideas and do's and don'ts in there.
Substrate also tends to be a high ticket item (I spent $175 just on substrate for my 90gal, and that was including some cost-cutting measures), but you can also use substrates that are inert (aka cheap) as long as you use fert tabs and/or dose the water column of the tank with ferts.
On the other hand, if you decide that you REALLY want to cut costs, you can set up a low tech tank. Low tech means that your lighting will need to stay around or under 2 watts per gallon, you can forego dosing the tank with CO2, and may not need to dose ferts in the tank at all, just rely on the fish bioload to support the plants.