I am actually making a sump for my freshwater 55 gallon. It's not as complicated as your sump would be, but Blazer is right, 5 gallon would be too small.
Making one out of acrylic or using a 20 gallon long would be a good way to go. When I set up a saltwater tank (in a few years hopefully) I plan on definitely building a sump. I would say you should definitely build one for your 55 saltwater.
In the sump, you would need a powerhead as your return pump to put water back in the tank, I believe Mag Drives are best, which one you get depends on the GPH
flow rate of your overflow. Unless your saltwater tank is predrilled with an overflow compartment, you would definitely need an overflow box to drain the water as well. I'm not sure about undergravel filters, but you can definitely put your protein skimmer, heaters, HOB
filters, etc. in the sump. You can also make a refugium compartment and a bubble trap for your protein skimmer. Your siphon will probably break if there is a power outage, but I think that is somewhat unavoidable. It's not a big deal, if everything is set up right, you shouldn't have any flood/drain problems. Plus, if there was a power outage, all the equipment would stop, so it wouldn't do the tank any good even if the sump kept going. If you're talking about a brief power outage, I think there are ways to get automatic startup of your overflow - I'm just not clear on how that works.
When I was doing research on sumps, I got most of my information from Melev's Reef site. It's very informative. Marc (who runs the website) posts all the plans and details to his sumps. He also has a section called "How does a sump work?" http://www.melevsreef.com/what_sump.html
I would read that if you want details on all the components you would need and to get an idea of how to start building one. His main web page is: http://www.melevsreef.com/
You can find links to all his sumps and plans and he even has plans for an overflow box. This is a great site and helped me understand all the intricacies of a sump and all that it involves. My overflow is going to be based off his plans.
Hope that helps!