That's going to be the biggest thing, there's a bit of a learning curve and a few different concepts to understand if you want to design a good sump for your set up, but if you're not comfortable with measuring/cutting/gluing pipes then you're probably best to stick with canister filters. It's not really that difficult though, just remember the old adage of 'measure three times, cut once'!
In it's simplest form, all you need is a suitable-sized old aquarium, a couple of filter socks, some ceramic media (or pond matrix is better again if you want to spend more) in laundry bags, and a suitable return pump.