"What keyboard should I get?"
I get this question a lot from new students.
As a baseline specification, I'd strongly recommend a keyboard with 88 fully weighted keys. Graded, hammer action keys would be even better, but as a first keyboard I'm happy if students have weighted keys. Other teacher's may have different requirements so always be sure to check with your own teacher, but for me, the weighted keys is the hill I'm willing to die on. Personally, I don't have a preference for brand, so anything you can find with the above stipulation is fine. New, used, rented, owned, it's all fine, just make sure it has 88 fully weighted keys.
What does "weighted keys" mean, and why should you care? Without getting too technical, it means that the response of the key when you press it will feel more like that of an actual piano and offer a bit more resistance than a non-weighted key. There are many reasons why this matters, but this post is geared toward early beginners through intermediate players and the reasons it matters even for them from a very early stage.
Pianists do not take their instrument with them to lessons or performances. If a student is practicing on a keyboard with non-weighted keys, it means that they will inevitably struggle with the instrument they play when the come to lessons, or go into any performance situation, because the keys on their lesson/performance instrument don't feel like what they are used to. You can go up stairs without watching where your feet go, because stairs are a standard height. You can even run up those stairs. Take away that standardization, and you will trip all over yourself. It's not a flawless analogy, but you get the idea and I wanted to choose something that I was reasonable sure most people had experienced, like going up stairs without your eyes fixed on your feet.
For young beginners, in the 4, 5, or even 6 year old range, practicing on non-weighted keys can mean that they struggle even to depress the keys on the piano when they come to their lesson because the keys on an actual piano offer so much more resistance. This is frustrating to say the least, and can slow down progress since I can't assign new material until I see that they've mastered the old material, and even if I were willing to give new material based solely on the report that it was working at home, ultimately I'd be doing them a disservice, because what is going to happen when they go to their first recital and are faced with a piano that has keys they can't work with? The fact that they could play their recital piece at home will be of little comfort at that point.
Once students reach an intermediate level, we've generally overcome hand strength issues, but now we have issues with speed, accuracy, and clean, crisp playing.
Students at this level often come in playing fast passages that sound "muddy". All the nots are correct, but they aren't necessarily getting released fast enough which can make one note bleed into the next. If they are practicing on a keyboard with non-weighted keys, cleaning this up is going to be tricky. Often, cleaning up a passage is a matter of releasing the keys cleanly. Here's the problem: A non-weighted key not only offers less resistance going down, but tends to be a bit sluggish coming back up again. A student practicing on non-weighted keys is not going to be able to tell if they are practicing correctly because no matter how quickly they release that sluggish key, it will behave the same. Some students might be able to practice this by "feel" alone, just by trying to to remember to lift their fingers quickly, but without the feedback of a difference in the sound they are producing, it's discouraging because they can't tell if they are doing it correctly.
Like I said I'm happy if my beginner students have a fully weighted keyboard for the firs several years. There is, however, and even better option. Graded, hammer action keys take things on step further to try and closely mimic the feel and response of an acoustic piano. The keys offer more resistance than non-weighted, but also, the lower octaves offer more resistance than the higher octaves.
If you don't want to worry about upgrading, then definitely go for the graded hammer action version. If you aren't sure this is an activity you or your child will continue, then it is fine to go for a less expensive version at first, but be aware that you may have to get a new keyboard in a few years.
Another option is to rent a good instrument for a few months, or a year, and then buy at a later point.