First of all: I'm not new to oblivion, so none of that, I'm a returning player.

Anyway, The first time I played the main quest, I jumped right in at the beginning, and was fighting stunted scamps exclusively.

I know that at higher levels there are loads of other monsters, but I never could wait.

The question is, what's a good level to start/ will have a good variety of daedra?

level 1 is the definitive answer since you start the MQ and game at the same time

also if you want a better chance of NPCs surviving do level 1

Gamrx is right that the best chances for having NPCs survive is level 1. However, since you're asking about levels to see a large variety of Daedra, the lowest level to see them all is 22. A peek at the right side of this table shows which Daedra appear at what levels: http://www.uesp.net/wiki/Oblivion:Creatures

Thank you, just was I was looking for.

Level up as far as you can before you start to encounter frost and storm atronachs in the world

You'll easily spank all the daedra, but once you start getting atronachs it becomes painful to go into oblivion realms.

I'm of the opinion that the first part of the MQ is best done as early as possible. You can...

I think the MQ ties together nicely with one or more of the guild quests. You have to run all over Cyrodiil anyway, with all the quest lines, so you might as well accomplish more than one thing while doing it.

As for the daedra getting more difficult, your character should be getting tougher as you level, too. There's no reason that fights should get harder. My current character is in her mid-teens level now. When she started the MQ it was taking her one poisoned bowshot and a single destruction spell to bring down a stunted scamp. Now it's taking one poisoned bowshot and a single destruction spell to bring down Clannfears and Frost Atronachs. The difference being that her Marksman skill, her Alchemy, her bow, her arrows, and her Destruction skill have all gotten correspondingly better.

I'm not concerned about difficulty, but I think it's boring to just fight stunted scamps and Dremora Churls.

I can understand that. I was just pointing out that the actual fighting doesn't really change much, if you're attending to your skills. The main difference is that, at high level, you'll have a hard time keeping your friends alive.

My character (Buffy) is permanently at level 19. Most of the Daedra she sees are associated with hunting conjurors, but at that level, she sees a fabulous variety and enjoys the differences and challenges that each provides. Only the Xivilai exceeds her level and is not seen in her game.

A picky footnote: Baddies actually can appear 2 levels before your character reaches the requisite level - but only along roads and as 'bosses'. This means that Buffy (level 19) certainly does occasionally see the level 20 minotaur lord for example. But Daedra as bosses are pretty rare and they are even rarer just wandering the roads so there's not much application of this pertaining to Daedra.

That's the mistake I made on my first playthrough, I worked on sneak, acrobatics, and armorer as much as I could early on. Kvatch was a nightmare for me .. actually most of the game was until I finally got blade and light armor inline with the rest of my skills lol

The game is so huge and with so many things to do that it is best to space it out.

Do part of the MQ before level 10 and aim to complete it in the 20's. That way you get to see the best of all worlds. I agree with the OP, completing the MQ at 1st level makes for a fairly boring quest, especially with all the back and forward journeys into Oblivion. Instead, it is cool to see creatures like the Spider Daedra appearing as you come close to the ending.