OK, lemme see if I can answer all your questions. lol
Yep, the heater is fine in the sump, thats where most of us keep them if we have a sump that is. You would be running you sump from day one, right from tank fill up all the way through cycling.
As far as the rock it's up to you. You can start with all base rock, a mixture of both live and base or all live. The base rock will become "live" as you cycle the tank and the beneficial bacteria multiply which then will "infest" the base rock LOL. The base rock will look really good after a few months too, you wont be able to tell the difference between the live and base after a while. If you start with all base rock you won't have any hitch hikers like crabs or worms or anything like that. That can be a good or bad thing, some people like to find the life on their rocks, some not so much as you can also get bad or destructive hitchhikers.
Also base rock is ALOT cheaper than live and ca
allow you to get all your rock at the get go instead of waiting if budget is a concern.
Using live rock and sand in the sump is also up to you. I have one setup., my 125g reef that has a refugium with sand, rock and macro algae. My 90g and 60g do not have that, just the equipment in there. I can't say for a fact if I have seen any difference between having a refugium or not having one. I can say I don't have any algae whatsoever in my 125g tank, but i also don't really have much in my other tanks either. Just a smidge here or there, hardly noticeable unless you really look. lol
The deciding factor may just come down to how much room you have for a sump and how big of one you get, there just may not be room for a refugium.
Hope that all makes sense, I tend to ramble on sometimes LOL .
Did you go see that 75g setup yet?