For her I would say a 125 two stroke or 250 four stroke. Lots of people will say get a XR200/230/250 but I say if she can ride an SJ, she can ride a real bike!
For you, well I wouldn't get anything smaller than a 250 two stroke. I would either get a 250, 300, 380 two stroke, or a 400, 426, 450, 525 ect four stroke.
Personally I used to have a YZ250 two stroke and loved that bike. Plenty of power, typical MX bike maintenance, decent parts prices, ect... I want another one but I think I will get a Yamaha (WR) or Honda (CRF/X) off road 450 four stroke and make it a dual sport. I plan to off road it more than on road it. Also electric start is a huge plus. It is so much easier to push the button then kick a new modern day fourstroke. As far as brands I think any Japanese (Honda, Yamaha, Kawasaki, or Suzuki) or KTM bikes are all good bikes. As long as you get a newer bike that hasn't been dogged, maintenance issues won't be all that expensive. They are like skis though and you can sink all kinds of money into them without getting it back. I see more expenses with all the tools and riding gear. You will absolutely need a helmets, boots, goggles, and gloves. I recomend knee pads, a kidnet belt, and a chest protector too.
Good luck. I am going down this road myself but just haven't fully paid off all my debt so I will have to wait a little longer.