Maybe you should give Elgg a try by setting up a test installation (could also be on your local computer) to get a better impression on what Elgg already offers by default. By adding a few of the additional plugins available here on the site you might be able to achieve most of what you have in mind already but it's better to test this out on your own. For specific custom work necessary you can add a topic in the professional services group here on the site. With making such a posting you will most likely get better replies regarding costs and time necessary for coding instead of only a rough estimation.
Thank you for your precise answer iionly.
I don't want to (and I can't) pay for the plugins, I asked how much time is needed to have an estimation of the feasability.
Anyway, I still need some help, because the doc is not sufficient for I really understand how to create my own plugins. (someone?^^)
Well, i finally found a book where i learn how to develop plugins : "Elgg 1.8 Social Networking".
Before i was completely lost (i think the doc is not sufficient). This book should be recommended on the site !
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