It sounds like you’ve just got surface rust. As others have said a good pressure wash of your wheel arches let it dry and follow up with a wire brush.
Your suspension arms etc I’d just clean up as best you and touch up the paint and follow with Bilt hamber UB.
If you want to be thorough and start stripping things back I’d clean back to bare metal and coat with an epoxy mastic paint. For the underbody itself you can touch up with stone chip paint for a factory finish.
If avoid the black shutz type under seals as they look a mess and don’t last long before trapping moisture and causing more rust.
The bilt hamber stuff is great value and diy friendly