Hello,
Instant ocean is fine to start with, Randy Holmes-Farley used it for years. He's the chemist that runs the Chemistry forum. I use the Red Sea blue bucket, mostly because it matches my tank parameters, it's cheap and has a bucket to store the salt
For the heater, get a controller. I use a finnex controller I got for $40, even good heaters malfunction. A controller has a separate temperature sensor and supplies power to the heater based on your settings preventing the heater from getting stuck on. Also sounds an alarm if too far out of range.
IMO, more smaller pumps is better than just one. You can arrange them to eliminate dead spots around the tank, flexibility you don't have with a single powerhead.
Here's a like to an article from Randy on optimal reef parameters
https://www.reef2reef.com/index.php?threads/173563/
Take a look at the stickies here in the new to the hobby forum and also the stickies in the chemistry forum... Lots to read and learn, just take it one step at a time. Any recommendations you get, think about and research before implementation.
Happy reefing!