What level would you make it? Apprentice?

Less than Frost Atronach but more than a Familiar

Master level, and I'd make the ice wraith it summons twice as big as a legendary dragon and do three times the damage.

Most likely apprentice, I think the Tel Nalta mod has an ice wraith spell in it, wonder what the mod-makers made the level as...

I think Requiem had one too, but it might be unique to Draugr.

They're kinda fragile but they can do quite a bit of damage, so I'd put it as an Apprentice or Adept spell.