I want to play - primarily as a mage - but a sneaky one that is invis when sneaking or doing a critical hit.
Can such a combo work or is it best to go about it as a mage and cast invis or stealth all the time? Got a good build?

thanks

Nothing's easier than being sneaky. Sneak can be worked up easily as either a minor or a major skill. However, to get good critical hits you also need a high weapon skill and high strength or high agility also help. All these things can detract from your magic skills. The critical hit bonus applies only to your base weapon damage, not to additional damage from poison or enchants. You could do something like:

Altmer

Magic Specialization

Blade, Restoration, Sneak, Acrobatics, Mysticism, Mercantile, Speechcraft

You'd primarily focus on Blade and Sneak as major skills. The minor magic skills, even Destruction, can be increased as you like. I would not advise using this build if you're interested in using a bow. There's a lot of other possibilities, of course.

Mara

I think this would be pretty good.

Race - any, use Mage sign for non magicka enhanced race, and thief or whatever for Breton or Altmer

Spec - magic

Blade
Destruction
Illusion
Marksman
Mercantile/Sneak
Restoration
Speechcraft/Sneak

Sneak is optional depending on preference. Mercantile and Speechcraft their for roleplay, and have the potential to level very slowly thus helping with game mechanics.

Race - Khajit

Spec - Stealth

Blade
Destruction
Alteration
Conjuration
illusion
Light Armor
Sneak


I designed this to be more of a theif build but it ended up being closer to a mage.

Race: Dunmer
Sign: Mage
Specialization: Stealth

Blade
Destruction
Illusion
Restoration
Light Armour
Marksman
Sneak

I usually play a thief/mage character and I always find this to work rather well.

You could go with a Nightblade, or even tweak it to your liking. I had a Dunmer Nightblade, and I completed the thieves guild, mages guild and the DB with no problem so it will probably work for you.

You can also temporarily boost related skills with fortification effects from Restoration spells or various potions.