Sumps can get a bit $$ but If you have the time you probably could build one like me. It only ended up costing me about $250 for a full 10 gallon sump w/skimmer but if you buy it already built it can end up costing $300-$600. They arnt hard to set up . A sump is pritty much a small tank that holds all you equipment such as protien skimmer,heater, filter media, ext. It is mostly used to keep all the junk that you don't want showing underneath the tank. The biggest problem wqith sumps is over flowing.
Bingo. Its the kind of thing where if you can't do it right you shouldn't do it at all. A sump gives you safety. There are less apertures in the lid to plug. Less ways for your octopus to get shredded. Larger water volume. Even most important is equipment. Lets be honest: most HOB equipment is garbage. HOB filters and skimmers are typically marketed towards the fair-weather, cheap-minded hobbyist because all the serious hobbyists out there (reef-keepers) who are willing to pay for quality have sumps.
How many aquariums have you had in the past? I recommend going to a local fish store (not a chain...) and just asking some of these questions. You will be able to see some of the answers to these questions for yourself. It is always a good idea to develop a relationship with a good local fish store, they can help you in many ways...
A sump is nothing more than a second fish tank that is plumbed into the first tank and normally hidden to conceal all the filtration garbage.
Think of a public aquarium display.
The trick to sumps is the design and safety features -these can make it or break it. The hard part in DIY sumps is the plumbing-this is not an issue with premade sumps. I would not recommend making your own sump until you have had one. The design looks simple when you understand the mechanics, but thinking it is will be the fastest way to soak your carpet.