I'd get the rock and sand first, that way you can start cycling the tank while you save up for the other things. Next I would get the powerhead, then the lights and finally the skimmer. An ac50 is perfectly fine as the filter, I used one on my 29g tank and it was perfect.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.