Like any patch, encompasses all previous patches: for full Changelog check description.
SSE does not change the ending of Tribunal, only expands upon it. Sotha Sil will still be dead, though there is, as we say, more to it. The only things SSE will modify is the end dungeon in Tribunal, and Barilzar's Mazed Band. Any mods that make changes to these will be incompatible, otherwise you'll be fine. If you have already beaten Tribunal and have the Mazed Band in your inventory, equipping it and teleporting to Sotha Sil will begin SSE, including the fights with the new Imperfect and Almalexia. Tribunal and Bloodmoon are required for SSE, though no other mods or plugins are. We still recommend Saint_Jiub's Clockwork City Reborn for textures, as SSE was built with it in mind. As for system requirements, if you are capable of running Morrowind on high settings just fine, you will also be able to run SSE. However, those with things such as MGE will be able to see more detail, as SSE has many interesting things rendered out of vanilla view range. All resources may be freely used, though if you do, please give credit.
Welp folks, we're here. The home stretch. After little more than a year in development, SSE will soon be ready for release, with all the goodies you've expected. In addition to a really really really big expanded Clockwork City (we're talking the size of Bloodmoon) we've got sidequests and crazed robotic NPCs galore, as well as three new sets of armor, a new weapon line, and a bunch of other things. Check em out in the screenshot section.