I would use standard plated bolts and coat the threads with Neversieze or similar product. Those products lube and protect the threads. I use it anywhere corrosion/heat is a problem like sewage, salt water
manifolds. When assembling an engine
, I use it on outside threads where rust might be a problem. I have removed bolts from large submerged sewage pumps that were treated with neversieze and the threads were like new while the bolt heads were almost gone.
You don't need stainless everywhere unless money
is no problem.