I wholeheartedly agree with the gift certificate.....a BIG ONE so she can have the machine of her dreams.
On the other hand, talk about her love of sewing and casually ask her about the "machine of her dreams." what would she really love to have if she could have a new one. I bet you will get some good responses, enough to fuel your energy toward buying her the one big gift she really wants.
On the practical side, I would NOT want my husband to give me a machine of his choice. He has no idea what I would want or use. Surprises are nice, but sometimes they go awry.
Better safe than sorry!
June