If you gift 20%, she can borrow 80% but I am unable to advise of rate or terms.
Is this for a co-op or condo?
I don't see any reason it wouldn't work. The drawback to being a co-owner is that your name would be on the mortgage along with hers. In addition to making you responsible if she defaulted, it would knock a few points off your credit score. But that sort of arrangement is done all the time. In fact, the first house my wife and I bought, my mother co-signed and is a co-owner.
You might also look into equity sharing. That could give you a lot of flexibility in how you can help your daughter.
To figure out how much of a mortgage she'd be able to qualify for, you'd have to talk to a good mortgage broker. A major variable, I'd guess, in addition to her credit would be her employment history and employment prospects.