I want to build a gravel bike for my wife using this frame, but I'm a bit worried about the sizing.
She is 158 cm, do you reckon 49cm frame is ok (even with 70 mm stem), or still too big and I should look for something different?

I am 99% sure that the bike is too large for her.
The bike is almost too large for me, even though I've built it as short as possible, and I am almost 10cm taller.
See more info in my build notes.

How is that stem for you? Considering going for the same.

Wouldn't recommend it, no. The compression ring is too short meaning that the headset cover couldn't clamp properly and I had headset play.
Ended up designing a C-shaped spacer which I 3d-printed and added between compression ring and headset cover and the headset play is gone.
The looong headset spacer you see on the picture is also 3d-printed by myself but is probably far too tall.
Will probably go down by 2,5cm when I dial in the rest of the bike.

That being said, I could share the 3d-files and you could have them printed locally, in which case the headset works great.

You should be fine with the wide crankset. There are numerous of mullet builds with road cranksets.

Even with the 55mm chainline of SRAM Transmission? Do I need to space out the chainring or similar?

Just finished my first ever build! I had some issues with a few little things but overall I am really happy with it. The rear brake needs to be adjusted and my handlebars aren't wrapped yet
Edit: I have attached a table with prices and weight if anyone is interested

The price of the frame seems MUCH lower than what I am able to find and your's even has a custom paintjob.
Could you share where/how you bought this?

Am considering building up one of these but am SO in doubt about sizing as this would be my first gravel bike.
Ride MTB normally.

Height: 167cm
Inseam: 76,5cm

Any insights on whether a size 49 would fit me? As far as I have understood, the toptube would be too far?

Thanks <3
Peter