15 should be a good level because the one enemy that you will have to fight is Level 20. I'm tempted to do a level 1 playthrough of it but I uninstalled Dawnguard so that's not going to happen.
Gotta be level 10 to start the line...
Anyway.. I'm playing it on a level 64 and the fights are not that hard, even on master difficulty.. but If i don't watch it, I can still get electrocuted.. None the less though, It has been very fun so far, and the battles are nothing like when I raid a bandit camp lol. I have been trying to only use abilites from schools I haven't quite mastered yet to make it more of a challenge, and have had to switch back for some fights.. not sure bout a level 81 though.
What i should probably do to make it even more a challenge is make some decent Light Armor since I'm 100 in Heavy.. that might make it much more of a challenge.
If you really love the Skyrim world, starting a new class can be fun, if you are looking for a real challenge, I would say probably anywhere level 10-60 on master difficulty would be best.. I would also suggest trying to discipline yourself not to fast travel alot.