Hi Wess, is this prototype still open? Your $90–140 budget works for me.
I'd start with the terrain and third-person movement so you can check the route and scale early, then add the sand/grass/rock/volcanic materials, ocean, simple building and volcano effects. Final delivery would include the source project and a full-island walkthrough video.
Would Godot 4 work for you, or do you need Unity/URP?
Here's a current 3D sample with the editable Blender source:
https://luohuashuang-chinese-classroom.alidonghao118.chatgpt.site/furniture