Your height is only one part of the puzzle. What are your other body measurements?

Tri bike or road bike? What manufacturer?

Sizing name conventions also matter. One manufacturer’s medium might be the exact same size as another’s large. For example, I have two bikes that are named as size medium by their manufacturers (Dimond and Canyon) that would be named as a size large by just about every other manufacturer in the world given the dimensions of the frames. My other tri bike is a large, and my other road bike is a 56cm. So I have four bikes more or less the same size that are called different sizes.

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Favorite Gear: Dimond | Cadex | Desoto Sport | Hoka One One

Been offered 2 bikes:
FUJI SE Medium size - 290 eur

and

FUJI BARRACUDA 105 groupset - M Size 52 - 330 eur

Would any of those fit me? Any better than the other? For absolute beginner?

What are the dimensions of those frames?

I think Fuji's name sizing runs on the small side. So their medium is kind of medium-small really. Like 52 or 53 cm.

I'm a bit taller than you (around 184 cm) and a 52 or 53 would be way too small. You'd likely be better off in the 55-56 cm bike range. That's without knowing anything else about your fit parameters but I'm pretty confident in saying anything in the 52/53 range it too small for you.

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Favorite Gear: Dimond | Cadex | Desoto Sport | Hoka One One

I’m the same height and a 56 cm

Don't go so much by S-M-L. Go more by top tube length. At your height you should be looking for about a 55cm frame size.
There are a lot of variables besides just height in fitting on a bike properly.
If the bike is slightly smaller you can adjust seat height and stem length. If the frame is too large there really isn't much you can do.