Converting my freshwater 80g to salt water

I have an empty 80g I'm thinking of converting to salt water. I know what I need in terms of sand, rocks, and lighting (I have a 4x65watt PC light for it), but I wouldn't know where to start for filtration. I dont want to drill it... someone once suggested a canister filter+powerhead+protein skimmer, but wanted to get advice from more than 1 hobbyist before I get anything going.

Please only answer if you are an expereinced keeper.

If you've already used the tank for freshwater fish, you probably already have much of the equipment you need to use it as a saltwater tank as well (filter, heater, powerhead?) as well as the lighting. I'll give the list I've been providing for people who ask about starting saltwater tanks, with some specifics for a set-up for your size tank.

FILTER: It's possible that you may want to just use your old filter if it's properly sized (filters at least 5x-10x the tank volume per hour - check the gallons per hour rating). If you need a new one, or are looking to upgrade, there are lots of choices, and what you'll need will really depend on budget, what you're trying to keep, and personal preference. I'd consider the best to be a refugium or wet/dry trickle filter, as these provide extra water volume for the tank as well. You can do one of these without needing to drill the tank, but it would better to have the sump above the main tank in this case, so you get at least one direction using gravity flow, but this will also mean a slightly lower water level to accomodate water in the plumbing if the electric fails. Having a second tank gives you more surface area for a biological filter, and you can use macroalgae to remove some of the nitrates. If you do a refugium with gravity flow into the main tank, you'll be able to produce copepods and small shrimp which will get caught in the gravity flow and provide occassional "treats" for your fish. If you keep the sump and main tank on opposite 12 hr. photoperiods, this also make the pH more stable (has to do with products of photosynthesis).