I am making a dungeon with no plot, quests, or puzzles. Just non-stop combat challenges. Enter the first room, kill all the enemies for exit to appear, move on to next room. Lather rinse repeat.
Because I want this dungeon to be extremely replayable, I'm trying to use all leveled enemies and leveled loot. That way the player gets fresh loot every time they enter the dungeon, and they get challenging battles no matter what level they are (thats the theory anyways)

I had read somewhere that Encounter zones don't reset. So that if a player enters a dungeon at level 5, then returns again at level 20, the enemies in the dungeon will still be level 5.

Is this true? If so, then I have tried to address this with encounter zones. My dungeon is broken into 'layers' where each layer has it's own encounter zone. This way, even if the player is level 5, the enemies get progressively harder with each layer. This is my current leveling setup:

Layer 1 - lvl 0-15
Layer 2 - lvl 20-50
Layer 3 - lvl 30-50
Layer 4 - lvl 40-50
Layer 5 - lvl 50+

My question is: Is this setup too agressive? I want low-level players to be able to have a decent time with the dungeon, but be encouraged to return later for greater loot and challenges. At the same time I want high-level players to have fun by being seriously challenged. I'm making this mod mostly for primary characters (e.g. the 'First' character you played Skyrim with, the one that has beaten the game, leader of all guilds, and can one-hit-kill Giants) but I also want to accomidate those secondary characters that may not be quite so high-level.

any thoughts/ opinions/ advice would be appreciated.

Great idea! You could also script enemies generation via Form lists, depending on the player's level when entering the area (trigger). You could even make the Form pointer random, so that you'l;l never get the same set of enemies for the same player level. That way you wouldn't have to think about zoning and leveled lists generation limitations, you'd keep full control over the process.