Digital pianos really have come a long way from where they were 15 years ago, when I started playing.
I started on an unweighted 61-key touch-sensitive keyboard (touch-sensitivity is, by the way, essential, but implied on the weighted keyboards. You cannot play classical piano music even remotely musically without touch sensitivity. Organ music is a different story). It's great for playing "Mary had a little lamb" and even "Fur Elise", but when you start hitting the more involved classical repertoire, you need to have weighted keys; it's simply too hard to play without the proper resistance. It's impossible to play more advanced classical pieces on this keyboard, because you run out of keys. The last major drawback was that it made playing on real pianos very difficult, since it doesn't let the student develop the musculature required for playing for long periods of time on a real piano (I'm only talking a half hour, here). That's because while they're often used as such, unweighted 61-key keyboards are not designed for use by classical pianists; they're designed for use as synthesizers. I still have this keyboard, and whenever I have to rock out a Katy Perry or a Taio Cruz, that's the instrument I turn to.
I then bought a piano, but from the sounds of it, the piano itself and the upkeep are way too expensive at the moment. I will say that I never bothered to keep it in tune (I've tuned it maybe 3 times in 10 years), and I regret that irresponsibility every day. That being said, it still works! The greatest drawback is the noise level; pianos are LOUD! With keyboards, at least, you can pop in headphones, and it'll be mostly silent (you still have to deal with the thumping of the keys, but it's tolerable).
My latest keyboard purchase was a Yamaha Portable Grand. It's an 88-key fully weighted keyboard. Keep in mind that fully-weighted is just what it means: it feels like a real piano and is heavy as anything (My estimate is 50-60 pounds; I feel that portable should have been in quotes on the packaging). It sounds beautiful and never requires tuning. This particular keyboard has something like 500 voices (which is actually useless; I never use anything except the stock piano voice, and occasionally an organ). This is the sort of keyboard you'd want for advanced classical pieces. The only major problem that I have with this particular keyboard is that the sustain does not last as long as in a real piano. This leads to sloppy pedal work, which does require a little bit of concentration to adjust for. The other minor issue is, as I mentioned before, the weight.
It's worth noting that not all fully weighted keyboards are that much of a pain; I am in particular a fan of Casio's Privia line. They're lighter, and I wish that I had picked that one up. I have not played with them enough to be able to comment on whether or not they have the same issue with the sustain pedal (It took a year before I noticed it on my own keyboard).
Finally, halfway between unweighted and fully weighted keyboards, you have half-weighted or semi-weighted keyboards. I personally hate the action, but I hate it less than I hate unweighted keys. I have successfully played somewhat intense classical pieces on them, and if it'll save a few hundred bucks on the cost of a new keyboard, it might be worth it. When it comes to picking an action, make sure your wife plays it and likes how it feels; that really is the most important factor. When I was buying a piano, my parents dragged me around to three or four warehouses full of pianos so that we could find one with an action that I liked. (By the time I found a handful of good pianos, my fingers were very, very sore!)
Lastly, you'll need to choose the number of keys your keyboard will have. Your choices will include 44, 61, 76, and 88 keys. More keys means harder to move (my 88-key keyboard is depressingly close to as tall as I am), but also means less technically limiting. That being said, I don't think that I've ever used all 88 keys on my keyboard at once. I will reiterate, though, with a 61-key keyboard, you will eventually run out of keys and it will make practicing advanced pieces harder.
Make sure that you get a footswitch with your keyboard. You will need them for pedaling on many, many pieces. I don't believe that springing for a "piano-style" pedal will make a difference for beginners; it's mostly useful for half-pedaling, which is a technique that comes along later. But there are a lot of inexpensive ones out there, so if you really want it, it won't hurt!
It sounds really obvious, but also make sure that your keyboard comes with its own speakers! Some professional keyboards will not have built-in speakers; they assume that you're going to use a keyboard amp, and so putting onboard speakers would add weight and complexity unnecessarily. To clarify, you really don't need a keyboard amp at this stage of the game, so steer away from these keyboards.
Good luck picking your keyboard. If I've left something out, let me know!
