Craigslist has some great deals.
As far as size, I feel its good to figure out what you really want. If you are looking at larger Angels, most Tangs or fish that require larger tanks, its wise to go that size first. If the smaller fish are what make you happy, then smaller will be fine. 75 is a great starter size, but it eliminates a lot of fish. Jumping up to a 120-180 opens a lot of possibilities and I've seen 120s go for 75g prices on Craigslist.
My 75 will become a reef, but I find myself missing the larger fish I had years ago. i had convinced myself that i would be happy with smaller fish this time around, so jumped at a good deal for my 75. I wish I had gone 180.