Partnering with designers (or ANYONE) is tricky. You sound like you have an existing product that you want "designed and cleaned up". The best UI designers (unless they LOVED the product idea) don't want to work on someone ELSE'S baby. Same is true for the best coders.

In terms of hiring designers-- I think it's like hiring salespeople-- most people who need them don't know how to tell the difference between good ones and bad ones.

As I hunt around for co-founders (I just stepped down from RescueTime a month or so back), I'm purposefully AVOIDING settling on an idea because I think it's best when founders co-author the idea together.