I agree as well. While there are some pro's to having a sump, it isn't going to be all that necessary for a FW tank that size. There are enough filtration options for a 50g that do not require the use of a separate tank. It becomes a better investment for VERY large tanks with large bioloads because filters for tanks that size are way up there in price and the extra water volume helps.
That being said, if you do really just have an extra tank right now...a FW sump is going to be pretty much the same as a SW sump. The only difference is that you wont have extra equipment like a skimmer and may end up using some different media. Quite a few builds and videos if you search online. There's also the wet/dry option, although I don't know much about how to go about implementing one...