i've done quite a bit of research on amitriptyline and i find it reliable if you have everything you need for the cocktail and most important, up to 24 hours where no one is looking for you for it to be be completely risk free, this is one of those methods that take a looong time for some people but it normally shouldn't take more than 6 hours at most. but not being found is key to succeeding with this method, every single case where someone fails is because they were found too early, doctors can force your body to function and completely empty your digestive system.
you need a lot of it so if you intend to do it this way then it should take you quite some time to stockpile enough amitriptyline for it to be lethal, you also need a lot of long acting benzos and of course an antiemetic so take that in mind.
and as for peacefulness, it seems quite peaceful indeed, not only is it stated to be very peaceful in the PPeH, some people have failed with it on this forum and they describe it the same way someone would describe a coma, they just went to sleep and the pain only came after they woke up in the hospital, absolutely no memories of even attempting. this is what made me want to try this method, you die in a very deep state of sleep.
i recommend reading the PPeH chapter on amitriptyline, it covers almost everything about it including doses and how to take it plus some interesting information.