I think it is a lovely idea that you want to build a nice small bike up for your SO. However, I don't think it is a good idea to surprise her with this bike. Size and fit is the most important thing and she should have some say in what is on her bike as well. If it were up to me, I would tell her I know she will love this new bike and ask her for input and she may find it really interesting to build up and pick a bike with your help.

It could be a nice bonding experience.!