Wow thats alot of questions in one post!
By the sounds of it you will have plenty of live rock and sand in there. How deep is your sand bed? The live rock will be plenty to filter your tank so there will be little need for mechanical filteration although it can't hurt as long as you keep the media clean.
I'm really not a techie so I'm afraid I really don't know what the filter you have is. As for the lighting... it all depends on what you want to keep in the tank. Do some research on the corals you like and see what lighting they will require. Don't bother getting lighting now if you think you will upgrade it in the future, save yourself the time and money and get the best lighting you can now.
Getting a glass cover will do 2 things. It will reduce evaporation and it will stop any fish you get from jumoing out. If you are looking at getting fish that are known jumpers, firefish for example then go ahead and get the glass cover otherwise it will depend on the amount of heat you're getting from your lighting. To be honest I wouldn't get a glass cover anyway, personally I'd go for an egg crate cover. You have to monitor nano tanks often so needing to do daily top offs is not a bad thing. I find it stops you getting lazy with the testing.
As for the filter/powerhead question, you should aim for a 20xtank volume per hour turnover so you will need 500gph of movement in your tank. This is to help your live rock do it's job and to prevent dead areas and stop a build of of detritus. It would be best to spread it out between a few sources but in a nano space is an issue. I would definately try to have 2 sources of movement though as most corals don't like the jet type movement a single powerhead produces. Check the specs of the filter you have and find out what gph it moves then get a small powerhead to compliment it for the other side of the tank. On what type to get...Tunze nano streams are pretty good but someone else could probably help you more there.
Your clean up crew should include some snails, crabs, possibly a shrimp. I personally would get a mix of Nassarius and Trochus snails with some hermits and a cleaner shrimp. Don't get starfish as they will starve in a new tank and shouldn't really be in nano tanks anyway IMO. I would also stay away from urchins in that size tank too.