Kinda depends on what you want to keep in the new tank. If you are looking to keep tangs I would wait it out until you find a 100+ gallon size used. BUT if you just want fish and some corals you could use the existing 75g.
I found my 180g tank, fully loaded, didnt need to buy anything else for $500. It also came with over 300lbs of live rock. It was the deal of the century to me, those types of deals do come along just have to be in the right place at the right time.
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.