I have been wanting to roleplay a Noble, but the class is what stumps me.
ALl that comes to mind is Speechcraft, and Mercantile, with Personality, and most likely luck for the attributes.

This will not really be a super combat focused character, but he might hunt a few days a week, so Marksman and sneak could be factored in.
But that leaves 3 spots open(Don't worry about leveling too fast, I use Oblivion XP)

So a more organized list:
Race:Imperial
Birthsign:Either the Thief(Luck bonus) or The Lady or Lover(one of them has a personality boost, right?)
Class:Noble
Attributes:Personality, Luck
Majors:
Speechcraft
Mercantile
Marksman
Sneak
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I'd say blade, Illusion and block... I'd drop sneak.

Blade, definitely. Its handy to be at least sort-of skilled in it. (But, a noble would only carry a dagger).
Ermm, like Ashwanderer said, Illusion. Take restoration too.
But, what Noble would be fantasmical at block? None.
Marksman
Mercantile
Speechcraft
Sneak
Illusion
Blade
Restoration.

No Birthsign boosts personality.

The Lord is a noble sounding birthsign.

Thats all Ill say for now, click link in sig if you want more of my building philosophy

Heres what I went through..

http://www.gamesas.com/index.php?/topic/1114633-so-i-need-some-help-with-a-role-build/

Ignore armour just use shield spells if you have to, it plays very powerfull early and late game, in the middle it may level up quickly and cause some problems though.
No guilds done as of yet, most Quests not involving dunmer or women are not taken.
Combat, uses illusion to avoid fights and even numbers out, then goes toe to toe fencing the last foe, Osmundas really is unbeatable he rarely sneaks, or causes sneak criticals.
I play him like Errol Flin or Johnny Depp in PotC, and roleplay him as a freakishly perverted impulse driven Alan Rickman.

this is a good build and also meets the RP requirements you specified. Thief might be good for sign. If you read the personality type for Mage it suits Nobles, and would add some more magicka, though you might not really need it if you get spell making (inherited spell altar) and make good custom Command spells of short duration which lowers the cost/skill factor. Might be fun character to focus on Thief/Luck and be sort mage/rogue.

roleplaying has absolutely nothing to do with builds.

what kind of things will you DO as a noble is the question.

how have you come into money?

will you linger around your mansion eyeing your wealth? meet with counts for banquets? travel around cyrodil trading your riches? discuss philosophy with mages?

what does a medieval millionaire do with his time?

certainly not take jobs to clean a basemant full of rats.

Ponder the air velocity of european swallows? Test their coconut carrying skills?

Otherwise http://www.uesp.net/wiki/Oblivion:Noble#Noble, its not what I'd go for, but if you want to have your PC like a NPC, do that.

My altmer mage Noble, was a young upstart. Drinking and carousing in every inn and bar all over the land. In fact, for the first five levels, that is all he did. But he was bored. He wanted a little more excitement. After an altercation with a very un-friendly npc in Skingrad, he wanted to get back at him. Must have been all the ale.... He killed the offensive NPC in a fit of rage in the NPC's house. Later that evening, he received a visitor, and he started down the long, winding road into being an assassin. His story is still being played, he has second thoughts now....