Er..... exactly the sort of thing I wear normally?
If you're starting at the bottom, you really aren't gonna be wearing straight off the runway pieces. The fashion assistants that I do know REALLY mix it up - high street, vintage, some designer stuff. This is out of necessity than anything else since you get paid pittance when you start. Plus, you're more likely to impress the editor with your fashion ingenuity rather than your ability to run up a huge credit card bill.
If you're talking about a Anna Wintour type position, I'd still would retain my mix and match mentality. Sure, I'd wear the big guns....but I'd wanna champion the smaller, lesser-known designers too...that's just me though.....
Don't really wanna get into magazines but working at Mixte or Ten would be fab....