it pretty easy as said, doesnt need to be complicated, or expensive as usually, just move your equipment you have in the main tank, to the sump.
i use a 10gal tank for a sump on my 55gal.
i went to lowes, they sell plexiglass exactly the right size to fit in the tank, if you want to make baffles. i did, mainly for noise and also wanted to put live sand in mine, baffles keep it from getting to the pump. but you could just do away with baffles.
only issue really is a 10gal isnt big enough for a lot of the 'in sump' protein skimmers etc. my protein skimmer, which hangs in the tank, is to tall for the 10gal.
plumbing not as hard as it seems. unless you want fancy. just a feed in and a pump out. either by purchasing a hang on syphon or drilling tank (maybe a little extreme to start) or you can do like me, and build your own syphon system. using just PVC pipe. little complicated but some good designs on the net, granted, the DIY ones do have some cons, such as cant see if syphon breaks, also hard to make them surface skimmers, which is nice to have.
i say go for it. if nothing else, it will increase your total volume, which is always a nice thing.
i use a quietone 2200 pump, but mine is located in the basement so has about 7foot head it has to life the water, you wouldnt need anything that big, if it under the tank. i think im pushing about 350gph, not measured it, but estimating. my syphon only at about half capacity.
pic of mine. but it is still in development stages, i change it bit as it goes. nothing fancy ir complicated. just houses the heater, some mechanicl filtration and a little bit of live sand, although i dont believe that essential, long as you have pplenty of live rock in the main tank. i just dont have enough live rock yet. plan on putting a couple mangrove plants in it, probably remove a baffle then, for space.