Proper heater(s) for 75g w/sump


Reefing newb
I'm slowly narrowing down my equipment choices (I haven't purchased anything) and am now looking to figure out what is appropriate for heaters.

I plan on doing a 75g tank w/sump (size is yet to be determined). I live in a condo that is very temp controlled - extreme low is around 60 degrees (in the dead of winter if i forget to turn on the heat) and extreme high is probably 75. Temp is almost always 70 degrees +/- 2 degrees year round.

A few questions -

1) Is it common practice to have multiple heaters?
2) What is a high quality, reliable heater that would be appropriate for my tank size?
3) What wattage do I need? I've heard 5w/gallon, but I've also heard much lower being used
4) I would like to buy a controller with an audible alarm as well if the heater malfunctions and the temperateure goes beyond a certain range. What do people use for this?

I run a 150w and a 200w heater.

I am approximately 100g of total water volume.

Stay away from aquatop heaters. They are really bad. The rest are comparable.

I have an apex contoller. It controls the lights, powerheads, return pump, skimmer, ato, 2 part dosing pumps, leak detector and heaters.
I see the Apex controller is about 800 bux - it sounds like this is something that would be good to get for an upgrade once my aquarium really gets going. If i were to just start out with a normal digital temp controller, would this be effective? I kind of imagine my startup process being ~1mo for cycling the tank, and adding a couple clown-fish (or some low-risk reef fish) and watching them for a few weeks or more to make sure that everything is ok. So it would probably be several months before I really started stocking my tank.

And when you say 100g of total water volume, I assume you mean your main tank + sump?

Correct, total water volume.
The 250 is my main and the 150 is my back up. Keep in mind, double redundant system.
Any quality heater will last. You just need to monitor it once and a while. Heaters should be replaced every other year.
Don't buy new. There are so many folks that are into this hobby and leave in a short time. It's very common. I got my apex on here for 300 bucks and it was 4 months old.