Let me prefix this post by saying that I'm not trying to "beat up on the new guy". However, you do have some issues with what you plan.
If your asking if could use your existing lighting and filtration could support a SW reef system, I'd have to say yes, but the system would be very marginal and very prone to problems. While you don't mention the exact lighting your have, I suspect it is far too low in quality and quantity to support most corals. Your filtration system using the Fluval 305 is a bit marginal, even for using it on a FW system. It's going to be extremely small for a SW reef. The bottom line here is that you will most likely need to upgrade both of these areas.
A note on canister filters on SW reef system. They can be used, but you do some things a bit different. First, you need to clean it often, especially if you don't have a skimmer. Typically this is about once a week. Second, you should not use any bio media in the filter. The biological filtration in a SW reef is done by the live rock and live sand. Bio media in a filter just creates another dirt trap.
As for a skimmer, again it is possible to maintain a tank without one, but not recommended once you get into a tank size much over 25 gal (about 100 L) or so. I would highly recommend a skimmer on your tank, and I wouldn't set up a tank that size with out one. If you shop around a little your should be able to find a hang on the tank skimmer that is no more intrusive that a hang on the tank filter.
As for the existing hood. If you have one of the typical FW tank hoods, you might as well figure your going to replace it, since you'll need to upgrade lighting anyway. If you have one of huge, thick, made of a lot of wood hoods on the tank, you should easily be able to cut it out a bit to accommodate a skimmer.
As for a sump. In a tank this size, it's nice to have but it's not something you must have. You still might one one though, since it lets you remove a lot of equipment from the display tank.
One final thought, as you can see, you will likely be upgrading most of the areas on the system. About all you'll have left is the tank and stand. You might want to price out getting a new tank and stand and starting from scratch or getting an all in one SW tank like a Red Sea Max.