What you have in mind sounds great.
Just make sure when you build the stand that you leave enough room for the sump, a 55g is kinda large and almost as deep back to front as the 75g. You'll hava about 6 inches left but you have to consider the 2x4's size as well.
You'll need a skimmer, I'd recommend a reef octopus one, pretty good quality and they have a huge selection in terms of price. You want to aim for a skimmer rated for twice the tank size, that will give you a great skimmer with excellent skimming ability.
I used dry rock on all my builds but included some live rock as well to help the dry rock to become "live". You could go with a 50/50 mix if you aren't on too much of a budget as live rock can be quite expensive, anywhere from $5 to $11 a lb.