I am very new to music; I mostly enjoy composing electronica on Cubase, driving few software synthesizers. When I search for a melody, I use an Evolution midi keyboard, a thing made of thin plastic with a Fisherprice action (it is that bad I cannot say “synthesizer action”).
Recently I decided to improve a little bit my playing skills: I would probably benefit playing with more than one finger, and reading music from others will expose me to chords, rhythm and other things I haven’t really studied.
I have to stress: I haven’t and I will probably not play any real piano: too expensive, too much space consuming, to much maintenance. So there is no need for me to have a digital keyboard that fully mimics the real acoustic thing.
My questions are:
1) Is there a real interest to have a fully weighted keyboard with hammer action? A friend of mine told me it helps him playing in rhythm, that the resistance of keys makes the thing more “physical”. But he was trained as a child on a real piano; I am 40 and I suspect my hands are missing 30 years of training, and my muscles, tendons and maybe even my bones will probably suffer a lot before I can play ˝ an hour in a row (by the way, is there even a risk of injury?)
2) Is it really needed to have 88 keys? On a real acoustic piano, the size of keyboard is related to the size of the harmony table, so I guess the sound would be different with a 61 keys acoustic piano, but I suspect the first and last keys are not so often used in popular music. In other words: is your C1 key covered with dust?

i've played 76 and 61 key keyboards, and running out of keys even occasionally can be frustrating. As far as the weighted issue, I see where you are getting at. The primary reason for people to get weighted keys is not because they need to "get used to" a real piano. It is because the weight and velocity-controlled keys allow for expression. Even if you never want to buy a real piano, the weighted action keys are important. You can choose between heavier action (like Yamaha) to very light action (like Roland). I imagine you should have no problems with the Roland (i personally find it too light, but many people like it).

i started with a 49 key keyboard and then moved onto a 61 key one with touch sensitive key, and then a digital piano (pf500). running out of key do make you feel bad, because you just couldn't play a piece you want to play in its entirety.

also, you might want to know before hand that playing on a weightless keyboard has a quite different feel from playing on a piano. i remember that after i played on my keyboard for a while, one day i had a chance to play on a friend's Steinway baby grand. all of the sudden, as i tried to play something on it, i felt my fingers couldn't actually play on it at all, such different feel! i struggled quite a while before i could even play the keys on it, of course poorly, uneven, and full of mistakes. so, after that, i reset my keyboard's touch sensitive selection to the hardest level, and made a conscious choice to play with more finger strikes and staccato action, which did help me when the next time i played on a real piano. so, depending on the purpose of your playing, you might not need a piano (digital/acoustic), but at least choose a higher end keyboard with at least 'touch sensitive' function so that you could make transition easier from a weightless keyboard to a piano if you ever need to.