Just to relieve SK for a moment here...
Just become your girlfriend is, well, a girl, doesn't mean she needs a women-specific bike. Actually, women-specific bikes are just different frames, with different measurements, and a lot of marketing. What she needs is a bike that fits, no? That will depend on the length and size of her various body parts relative to each other, of her ease on the bike, etc. Just because she has not ridden in a while doesn't mean she'll be into comfort or low-traffic or racing or something else. This suggests playing it safe in getting a "simple", not too expensive first bike, yet good enough (avoiding cheap, frustrating components if possible), giving her chances to experiment and to decide what she really wants for her second bike.
I understand that doing things yourself might be a big difference in cost in the end, especially if you have parts at hand, plus: you're pouring a lot of love in the project which I am sure will be noticed

but I would suggest getting help from people who know their stuff about fitting, and getting her as involved as possible in the process, so she really feels that it's HER bike, and it really fits like it's meant for HER (not for the "generic woman" out there, whoever that is).
Thanks for doing this, we all wish we had a special person building a bike for us.
ETA: Although at this stage I'd rather build one myself.
