Let me try to address this:
A sump and a canister are not the same thing. A canister filter literally filters the water using sponges and media. A sump adds additional water capacity and a convenient place to house equipment (heaters/skimmers/etc), live rock, macroalgae, and other things too.
Generally, canister filters are frowned upon because the sponges trap detritus and create nitrates. They require constant, and consistent, maintenance to keep the nitrates down and the media "charged". Now, canister filters are very convenient for running carbon and other media, but there are other ways to do that as well (such as a phosban reactor). I have a number of idle canisters, but I don't use them on my tank unless I need to.
As for making a sump under your tank, your best bet is to put the largest tank you can fit under your current display tank and that will be your sump (I have a 20g under my 75g display right now). You don't "need" another cabinet.
As for sump design, check out
Melev's Sump Designs for lots of info. But if you use a glass tank, like a 20g you get at Petsmart, then you silicone baffles into the tank to create the compartments. HTH
