http://www.uesp.net/wiki/Oblivion:Sardavar_Leed - Tried this at level 15 as a sneaky type character with moderate skills. A "guardian of oblivion" toasted me inside the entrance way. Apparently it has been beefed up by OOO. I suspect I'll need to be level 25-30 before coming back and tackling this dungeon.

Yellow Tick Cave - Another attempt at tackling a conjurer cave at level 15. Now, either my level is too low, my sneak is too low, or conjurers are exceptionally aggressive - because my second pull resulted in a train of 6 or 8 conjurers along with pets. So I probably won't be able to tackle this until I have potions capable of silencing and killing conjurers.

Belda - This one is doable at level 16 if you pack the right potions and equipment. You'll need about 50-75 silence poisons (Motherwort Sprig, Nightshade, White Seed Pod, and Sacred Lotus Seeds), another 50-100 regular poisons, lots of health potions, and some cure disease / poison / restore attribute potions. If you're a bow user, you'll need an enchanted bow that you create in the Arcane University. My first bow is typically a silver bow enchanted to do 5 points over 1 second of three damage types (fire, frost, shock). With a greater soul gem (1200 points), you'll be able to get about 80 shots from the bow before you need to recharge it. Lastly, you'll need a few soul gems and at least 100 arrows (200 would be better). I also carry a sword enchanted in a similar manner to the bow (4 points of damage over 2 seconds for fire, frost, and shock). There was still one "very hard" chest deep within the dungeon that I couldn't open yet (so I'll have to come back). And some of the traps on the chests can kill you instantly if you only have ~150 health. So save frequently.

One of the hardest areas is towards the end (but not the last room) where you may face a Plane Summoner, who is a tougher then normal conjurer. Not to mention all of the iron / gold / mithril / other golems that take 5-10 shots from the bow (silence poison is useful, otherwise don't bother) to put down. Make sure you have lots of room to back up when fighting those, because they hit hard.