According to the U.S. Food and Drug Administration (FDA), the lethal dose of codeine is 0.5 to 1 gram, or 500 to 1000 milligrams (mg). Having underlying conditions, such as COPD, age, weight, etc *might* exacerbate the effects.
SN is not that easy to get right now due to all the news stories about suicides from it. Governments have tightened restrictions on who can buy it (legally), and beneath-the-radar sellers are few and far between. Whether it's "pleasant" is kind of subjective. To me any vomiting is not pleasant. It takes some other drugs to make it most effective, also.
There's a "MegaThread" on inert gas that includes Argon. It's link can be found in the Resources Compilation "sticky" that is located right above the first page of this forum. If you can't find it and would like to read it, I can link it, if you give me the word.
Inert gas requires some equipment and money to purchase it. You need a tank of the gas, a regulator for the tank, a flowmeter, hose, various fittings, a "hood", or a "mask", of some type. It has to all be assembled. I *think* it's a relatively peaceful way to go. Can't know for sure, though, as it's pretty impossible to ask someone who has done it successfully.
My neighbor and, also, my good friend, had a hip replacement when he was 90. He put it off for years until he couldn't take the pain any longer. He did extremely well with the surgery. As a matter of fact, he told me his doctor called him the "poster patient" of everyone who (the surgeon) had ever operated on. He's passed-away now, but after the surgery, he was back to his old self in no time. No doubt he wasn't an "average" guy. I even saw him "hop" over a fence into his cousin's yard (gate was locked) behind my house when he was around 88.