Choosing Materials (metal alloy) for DIY exhaust system

[OnionTou]

Assuming you had a shop with the right tools to make exhaust systems, what kind of metal alloy (and grade) would you use for making:

1) Header
2) Overpipe
3) Frontpipe.

Does the material affect sound or vibration (assuming you have the same resonator/cat converter)? I think thickness also affects the end result?

Any suggestions on material thickness?

Before someone rushes to tell me "stainless steel, duh!", Stainless steel has multiple grades in which the alloy is slightly tweaked, so I want a more complete answer.

While the idea of getting an already made product from a reputable manufacturer is always an option, I'm still looking at alternatives. Most products for this car are found in the US, but importing them to Peru puts a hefty sum on shipping and tax payments. Usually between 50-80% of the items price in the US.

[Gunman]

Inconel 625 if cost was not an issue.

[CoupedUpSubie]

If corrosion is an issue in your area 304 stainless, otherwise 409 is fine.

[ToyoburuBRZ]

I've used 18 gauge 304 stainless on all the exhaust systems I have ever built.