I have 2 ideas for new characters

The first - 'Honed Warrior'
Will be a tank, relying on melle combat. Uses alteration, restoration, and conjuration (Defensive magic), in addition to blunt and probably heavy armour. May use a bow from time to time but it wont be a major.

The basic ideal behind this is that he/she is to be the perfect warrrior, impossible to defeat, ultra tough and strong, and with the ability to resist more damage through alteration or recruit allies through conjuration.
The restoration has just always been how ive healed my characters but i am a bit worried this character wont have vast amounts of magika to spam the neccessary spells in this.
My prototype is an Orc, as it has a decent distribution between strenght, intelligence, and endurance, as well as a resistance to magika. Born under the mage birthsign to increase his magika,

The second - 'Fast man'
Inspiried by my current character, who is an archer, and my recent appreciation for the speed attribute.
He will most likely try to avoid combat, but when it occurs he will use his speed to never get hit, zipping in and out of combat with a bow or precisely timed sword rushes.
I will probaly tie this one in with sneak & poisons, because it just fits in well.
Magic wise im thinking Conjuration and/or illusion. My problem with illusion though is that most of the spells dont work if your wearing any armour, which is very annoying, so it woould really just be used for invisibility so i can run away, and in which case i may just stick to alchemy.

So, thoughts/improvements/criticisms on said character ideas?
What difficulty level should they play at so the game is challenging throughout?

On your speed build, you could just not wear armor and concentrate on speed to avoid combat, supplemented with a few articles of clothing enchanted with shield. That way you could make efficient use of illusion. I am playing a similar character right now on the Xbox (although without the focus on illusion). I am playing at default difficulty, which has honestly been pretty darned easy so far. I expect it may get a little more difficult at higher levels.

When I play on PC, I prefer to control difficulty with mods since the difficulty slider in the game is pretty limited, and with mods, you can do things like improve AI so enemies fight more intelligently and increase damage for both PC and enemies, so you can die in one or two hits but you can kill quickly too, so battles don't last an eternity.