"Is there any reason to go for 275N"
Yes. Simply put, the more buoyancy you have, the further above the water your face and nose will be. You CAN drown from inhaling wind-blown water mist and spray in rough weather, and every inch you get further above the water helps. More buoyancy, less water inhaled.
And in the event that someone else is in the water with you, you will find that having enough buoyancy to support both of you is a very nice thing as well.
"Hammer" is the job that puts a spring-loaded flap over the whole mechanism, so it stays drier until submerged. That's better than a plain aspirin tablet, although I take issue with them for calling it a "hydrostatic release".
I haven't hread of paper disks but Crewfti and others use a spring-loaded piercing needle which in turn is held compressed by some papier-mache wrapping. Once the papier-mache gets soggy, the spring bursts it off and the needle pierces the cylinder, inflating the vest. I've had and used one for years and from what I am told, it was more expensive than the "aspirin" but far more reliable against false alarms. I confess, I added a wrapping to make sure it wouldn't go off except when submerged--as opposed to any conceivable splash.
But the bobbins have always been harder to find (in the US) so the Hammer may be a newer better way to go now. Still has a weakness, if there was any moisture inside the head, that aspirin tablet is going to get soggy and inflate. Either way, travel with at least one spare repack kit.
Whichever you get, if it doesn't have crotch strap or leg straps, it is useless because it will ride up on you when in the water. Use the straps! Practical Sailor tested a number of vests some years ago, with any of them whether you float "face up" is a matter of properly adjusting the vest, and having enough buoyancy. Some foul wx gear has little tabs on the front, that fit the toggles found on some PFDs, so they position correctly. (Crewfit and Henry Lloyd used to do this.)
You may want to consider taking your PFD and cloths into a pool or pond, inflating it with the oral inflator if you don't want to waste a repack it, and see how it fits and works. Then rinse with fresh water before repacking. Much easier to trust it once you've seen how it works and floats you. And of course, that gives you a way to inflate it and let it sit for 48 hours, to make sure there are no leaks. Even a new PFD can be DOA.