There will always be a fascination in deadly plants and many of the most deadly turn up time and time again in herbalism from the exotic to the mundane. Like many I would advocate extreme caution when working with or even growing such plants.
Take the apple seed for example firstly the it would depend on
How the seed was taken, crushed, whole, dried etc.
The variety of apple.
The age of the seeds.
The amount of nutrients take up by the tree.
There is no way to know for sure the lethal dose of plants as the amounts of toxins change from plant to plant, time of year and even where and how they are grown. You can go down the lengthy path of extracting the toxin and then know the amount but this is not something easily done in a kitchen and most are very fragile and are easily denatured.
I grow over twenty types of 'deadly' plants in my garden. Among some of the most deadly is the humble kidney bean Phaseolus vulgaris and Monkshood Aconitum variegatum While Monkshood is a gorgeous plant and the kidney bean when cooked properly a tasty food I would be very reluctant using either in a spell.