Frame – How tall is she? I’m 5’9 and get on fine with mens frames but if she was small she might be better off with a womens frame – although anything small and light ought to be ok with a shorter stem.
saddle – anything with a groove or a hole. Unfortunately you cant really say how one person will get on with a saddle. I have had some womens saddles that are really wide, make it harder to pedal and hard to get off the back of.
you might want to think about brake spec – I found V brakes useless because I didnt have the grip strength (do they even exist anymore?). Having said that you will prob want brakes with adjustable levers (smaller hands, smaller reach). BUT she prob wont need massive rotors…
Grips – again hand size is a factor. I like ones with less padding / rubber.
Fork size vs stem size – she is going to find it harder to keep the front down when climbing than you do.
As for build – I did a similar thing for my BF a few years ago. Bought him a Soul frame and then put basic bits on. As they wore out or he broke them he replaced with more expensive bits…I thought the frame more important in the long term and as a beginner he wouldnt notice deore shifting quality. The Soul has been great and for the same money (once built) a full suss would have been hideous.