Kerrine, I'm also in the process of setting up a 75gal tank, and here's some advice I was given on this forum.
Filtration: for a tank that size, it was recommended that I get two filters. So I ended up buying a Fluval 404 cannister filter, and a Penguin 330 Bio-Wheel power fitler. Bought them both over the internet at a huge savings over the Pet store prices. (It was like getting the Penguin 330 for FREE!)
Heater: again it was recommended that I go with two rather than just one, so I'll probably go for a pair of submersible 200Ws.
Cycle: I was also planning to go the fishless route, but have since found a LFS
that carries Bio-Spira. The 3oz size (about $20), will treat a tank size up to 90gal. Seems like a bargain to me to get an instant cycle!
Substrate: I read somewhere that you need a one to one-and-a-half pounds per gallon. So I'm going to start out with three 25 pound bags and see if that gives me the coverage that I need.
Other stuff: for large tanks it seems like most people highly recommend a Python siphon system (or a similar home made equivalent), so I picked up one of those as well. (Again, online it was about 40% cheaper)