How to access to the user info on the search page?
Did you try implementing hook_preprocess_search_results (notice the plural search results)?

I haven't tested recently and I'm on a mobile right now. But I'm pretty sure that hook somehow had node objects (should be user objects in your case). Collect the UIDs, run a user_load_multiple and then add the user objects to each result for profit in hook_preprocess_search_results.

Update

I'm no longer on my mobile, so I could actually test this. It turns out that the user module does not feed the results set with the user account. I don't think Drupal 7 will receive these features anymore, so you'd have to patch the core user.module to achieve that. 

Here is a patch if you are interested in going down that path: https://gist.github.com/Ayesh/b1892942b053dc746fb5