Suggest you contact Stormlake to confirm if they can supply you with a conversion barrel to put .40 barrel in a 9C 9mm slide.
They list 9mm in .40 conversion barrels.
i thought you could barrel swap from the 40 to 9 or .357 sig but i do not think you can convert the 9 to 40. easy enough google search would answer that. its why i bought the M&P .40 just because i can switch it to 9 if i just wanna shoot for a more affordable price point. i shoot and carry it in 40 but its nice to have options
You can convert the 9mm to .40S&W. With a factory barrel it requires some modification to the hood because the .40S&W is wider than the 9mm.
I don't know about after market choices.
There is no reason not to use a stainless barrel. The only difference is color. Besides, you're going to have this gun for a long time. What's another 30 days to get it in black if you want that?