If your not gunna have a sump than you will need a hang on skimmer, No internal skimmers. I can recomend a few good brands of hang on like coralife, aqua c, and deltec. But i will recomend either the aqua c pro with the mag drive 3 or the deltec model MCE300. Because they are perfectly sized for your tank and are very slim so you can still keep your tank tucked in a corner. http://www.deltecusa.us/proteinskimmers/mc500.php
For the question about lighting, this can be a never ending argument but ill give it my $.02. If your using t-5,pc,vho, ect than the watt per gallon rule is exeptable for detirmining how many watts you should have. But if your using metal halide than the wpg rule is a little less usfull. to try and answer your question yes you could use 420 watts of t-5 lighting over your tank but you could also use one 250 watt metal halide pendent and two actanic bulbs(dont know the dementions of your tank so i can recomend a wattage) But the wattage will still be under 420 watts but you will have better lighing with mh than with 420 watts of t-5. Caferacer mike is stetting up a 400 gallon reef tank heres his lighting settup.
''4 400w lights in Lumenarc reflectors and 6 72" VHO retro kits''
6 72 watt vho's is a total of 960 watts of lighitng, with another 1,600 watts of mh lighitng witch equals out to somewere around 2,600 watts of lighting. Thats alot! but as you can see its hes not following the wpg rule cause if he did than he would have 4,000 watts of lighting. So the whole idea im triing to get through to you is the wph is not that great. If i were you i would build my own wood canopy, buy a 250 watt mh and use those lights that your tank came with as actanics.