I'll take a stab!
First, empty and clean the 72, if you Ever treated with copper in that tank you must remove it completely. There is post on that somewhere so I'll wait on that for now. If you don't have copper, clean the tank thoroughly, vinegar with RO/DI water is a good start. One thing is location, I'm sure you thought about that but SW & reefs may require some though as compared to FW, lots more equipment etc underneath. Some of your equipment like filter, lights can be used for starters but eventually they will need upgrading, to LR & Skimmer for example, your filter can be later used as a polisher filter for a few days then removed until next time needed, the media will also be limited to carbon & maybe phosphate removers but we talk about that later. Your filter will require weekly cleaning as to prevent nitrates from building up too high, although fish only can live with a small amount. I don't know if you've been reading but lots of people have been going bare bottom or CC thin substrate for the reefs. Fish only can be a little different but basically the same. I would use your LR and sand if you wish in the 72g. If you don't have Live Rock, I'd get some for bio-filtration, usually rule of thumb is 1.5lbs to 2 lbs per gallon, depending on the density of the rock itself purchased, it can be added little by little if necessary, just make sure it is cured if your adding to a tank with inhabitants in it. LR will be your main filtration alone with a skimmer. A good skimmer would eventually be needed to add to your filtration, better lighting & sump eventually. Don't panic, instead we have all the information you will need right here. Start off slow and lets discuss your plans first, then from there we can get you started in the right direction. Instead of looking at all I'm saying in a discouraging way, look at it as something that will be so wonderful in the long run, also with the combined experienced we can help you save tons of money and get you started on the right tracks. You can start with what you have and slowly work up to improvements, as not to break the bank. There is a few things in precedence you should get first, so instead of jumping in full force, take some time to get to know us & read up a little so you can ask the good questions like your posted here. We can start with getting LR from there, skimmer, lighting etc.. If you go reef one day you will also consider a method of keeping your calcium up, along with other water parameters in a balanced form but that can be saved for later If you ever go reef!
P.S. If I didn't say so before, Welcome!
Take off your shoes and stay a while!