No, this (as for now) has no influence on your profile and is not visible to other clients.

Yes, you must state all Textbroker earnings (impartial to the amount) in your tax return.

No! All texts must be a unique work and can not, in whole or in part, coincide with any published material. This goes for other as well as your own work.
As an exception you may use quotes if necessary. If you use quotations be sure to label them as such and mark them with the appropriate reference. Furthermore, only short passages of a text may be quoted. An adoption of several sentences or even an entire passage is prohibited. Also, you should use as many sources as possible for your article and always compose texts in your own words to guarantee a unique style.
Hereby please bear in mind the copyright laws as well as our Terms of Service. A violation can lead to serious legal consequences should a client or the owner of a concerning copyright learn of a n illegal usage of protected material. Incidentally this also counts for Wikipedia texts.