Guess what! UPGRADE TIME

[daplatapus]

First off, your existing tank looks amazing. Very good use space. I have huge respect for people who can pull that off with a smaller tank. But I was thinking about your new tank dimensions. My 77 is 16" front to back and I struggle with aquascaping to find a sweet spot between placing rock work and leaving enough negative space for glass cleaning and visual appeal. If the space you want to put your tank allows for it, you might consider a bit wider tank.
You mention that you want to use your existing T5 lighting and are concerned it may not be enough for a larger tank. If you went with an 18"-24" tank and put your rock work either centered or a bit back of centre, then mounted your light right above it, that would give you nice visual negative space with your rock work highlighted by the light. Place all your high light requirement corals on it, and low light or non-photosynthetic corals on the sand bed, (like dendro's, gorgs, or blasto's)
Then when finances become available, you can upgrade the light and you've still got a really nice dimension tank.