I have never been to Costco so I can say nothing about them. I have two Brother machines and love them both. One is a little cheapie from Walmart. It has many stitches and has sewn everything from curtains to prom dresses. It will sew through anything and has never seen a repairman. The other is a 1500S bought online. It's straight stitch only and I use it every day for piecing and quilting.
I think your gift will be an excellent starter machine for her.