Different keyboards for different usages.
How many keys are acceptable for a keyboard? What about weighted keys?
Cheap keyboards depending on your needs.
When you start learning piano, you will not use all the keys directly. The chances are you will first start with only using the middle 24 keys for quite a while. However, if you plan to play advanced classical piano pieces, the chances are you will not have enough keys. Learning piano, the first 6-8 years, you will likely only use the central 48 keys. If it’s pop songs you plan on playing, chances are you will never use more than 61 keys.
Can do without: the first 5 years
Tip: Most keyboards have an "octave transpose" option. This will virtually shift the keys so you can access the notes not physically present on your keyboard (but you loose notes on the other side of the keyboard, virtually).
Real piano players consider a keyboard without weighted key a toy. Since it just plays a pre-recorded sound when you press on a key, it kind of is. When you start playing piano and focus on playing the right note at the right time, weighted keys is a feature you will likely disable (even if your teacher tells you you shouldn’t). However, some basic piano techniques require that you have weighted keys.
Even you will soon tell that it sounds "flat", that there are missing dynamics. If your goal is to learn a few piano songs to show of to your friends, you don’t need weighted keys. It will sound better to you but since everyone will be singing along and having a good time, they won’t notice the difference.
Can do without: the first 6 months
Difference between weighted and touch-sensitive: weighted keys react as a real piano would where you have to push hard on the key to make a sound. touch-sensitive means that the harder/faster you press the key, the louder the sound.
Tip: weighted key usually also mean touch sensitive. However, some manufacturer like to play with wording and make weighted keys that are not touch sensitive.
On portable keyboard you always have speakers. The problem here is that someone learning the piano is annoying to listen to (sorry). I recommend you check if an audio output is available to plug in a headset (or plug into the stereo). This can also be good as the cheapest keyboards have bad quality speakers.
If you are alone and don’t mind bad quality sound, you can do without.
MIDI (and other outputs)
Even the cheapest keyboard usually have a midi output (sometimes on USB rather than MIDI connector). This midi output can be used to connect you computer to your keyboard. This is useful if you want to record what you play or have your computer generate the piano sound (which can sound a lot better). Your computer can also add effects.
Cheaper keyboards usually include many voices (guitar, drums, helicopter, etc.). As the price goes up the number of available voices goes down but you have a better sound quality and better speakers. It’s fun to take a break and play a "choir vs. helicopter battle" but it’s not really needed.
As you improve your technique, you will at some point want to use the pedals. To be frank, I only used them for fun to make the piano sound different. So I can’t really comment on them.