I want my dwarves to live miserable, danger ridden lives where I am forced to make food out of seeds. What is the best world/embark to do this on?
I guess you could try to make it harder for yourself by getting rid of caves via worldgen parameters, assuming that is even possible. Food and wood is generally plentiful in caves and that is only poorly counterbalanced by the hostile creatures that live there. Additionally, look for a biome with no useful vegetation above ground (I'm thinking glaciers and possibly badlands). However, even seemingly going over the top like this is hardly going to make your life difficult as long as you have your shit together. Regarding food production, you have essentially two alternatives: easy and impossible (if you were to embark somewhere without vegetation and water [or if you have ice, magma to melt it]), there is no way of making it difficult and having to rely on imports is only going to slow you down somewhat.
I don't see how the part I underlined makes the least bit of sense. That would be cutting off the branch you're sitting on, seeing how easy agriculture otherwise; by cooking seeds you are throwing away potentially unlimited food. While that isn't a problem normally, but assuming you want a difficult embark spot I assume those seeds would be imported.
Regarding monsters, I will soon be doing a bit of testing of my own as I've modded cyclopes, giants, and minotaurs to form hostile civilizations. With the new combat model I expect those to actually be pretty dangerous. Other than that, there are mods that introduce new sieging civilizations and I recommend either using them or looking through their raws for inspiration.