I have researched various digital pianos as I have purchased two in the last five years. I have also been taking lessons for over two years. For beginner piano, you don't absolutely need 88 keys, but I would recommend it. At minimum, 61 keys but you may end up wanting 88 keys as you progress. Finally, you want the best weighted key action you can get for the money.
Here are the different levels of digital pianos that I came across during my research:
- First, you have your really cheap, non-weighted (or cheaply weighted) beginner pianos that typically have built-in tutorials. They are in the $200-400 range. My recommendation is to avoid these period.
- Up from those in the $500-700 range, you have your basic digital pianos, made entirely from plastic with "grand action" weighted keys. They include 10+ different sounds, sustain pedal, MIDI, a built-in audio recorder and that is about it. This is what I purchased as my first and is what I recommend for beginners. I had a Yamaha P90 and it worked great for beginner to early intermediate piano.
- Up from those, you have glorified versions of the previous model. The key action is about the same, but they come in much nicer cabinets (wood) and probably better speakers. I believe they also come with a pedal board with all three pedals. You only really need a sustain pedal for beginner-intermediate level. Some may also have built-in computer for lessons. Price range is probably $1000-2000. I wouldn't waste my money on these.
- After that you get into the higher end digitals. The top of the line pure digitals that I looked at were Yamaha Clavinovas. These are in the $3000+ range depending the features. The models with built-in computer and instructional software are as expensive as $6000+. A piano in this range probably has the best grand piano key action for a purely digital piano. They also have high quality pedals that feel like a real piano. In my opinion, you at least need something like this or a real piano if you are going to get really serious. You won't be able to get into advanced material on a cheap keybed. You need the weight to build proper strength in your fingers. I now own a Clavinova and I love it. Mine is just a standard model without the computer stuff built in. I added a couple upgrades like better speakers, wood keys with synthetic ivory tops and polished ebony finish.
- Finally in the digital piano world, there are hybrids. They have digital sound source, but use real hammers, dampers and strings to trigger the sound. These are the most expensive, getting into the $7000+ range. Of course these are going to produce the most accurate piano action. These come in both upright and grand piano format. This is probably the only type of digital piano you can get with upright piano key action. All digitals I tried have grand piano action.