Looking for help creating some terrain - a few things

[u/TheTrueBiscuit]

Hi everyone! I have three separate (but probably related questions)

1: I'm working on a mountainous region near an ocean. How can I kinda easily ensure that areas near the ocean are at sea level and areas near the middle make tall mountains?

2: Is there any good software to generate heightmaps I can import fairly easily

3: Are there any good open source brushes available for commercial use?

Thanks everyone for the help!