The best certification, when looking for employment, is Red Hat (RHSCA + RHCE as a start). It's the industry standard certification if you are looking to be employed in a Linux position at large businesses. That said, indeed you can not get Red Hat certification in Iran because U.S. law prohibits services export to Iran currently. So your best option is to go for the second most recognized certification; LPI certification (LPIC-1 + LPIC-2 as a start). As shared, this will give you broader experience than just Red Hat, which may be preferable for employment opportunities. Certification for that is available in Iran I think (can't read the page
Linux Mint is an awesome desktop Linux, but when considering either Red Hat or LPI certification and future employment I recommend you get hands-on experience with Linux for servers. For employment at large businesses (worldwide), those will probably run Red Hat, SUSE or similar enterprise Linux distros which have paid support from their vendors (this may not be a consideration in Iran, given aforementioned export restrictions prohibiting that). Though web servers for example are most commonly run on Debian.
Debian is available for free, but Red Hat isn't. Red Hat's software is though, and both Scientific Linux and CentOS repackage the software for a near complete Red Hat experience. Scientific Linux gets the better reviews currently, as it is more up to date amongst other reasons. I'd recommend Scientific Linux. You may also have heard of Red Hat's community edition Fedora. Skip that; it is a "testing grounds" for new features that may eventually make it into Red Hat. Not relevant if you are looking for employment.
Downloading may not be an option for you. You can buy Debian, Scientific Linux, and others in Iran though. I found the following to stores that will sell you the discs:http://linuxshop.ir/http://shop.sito.ir/
Which should you pick? I'd perhaps contact the LPI in your country, explore options and cost for training and certification, and perhaps also ask them for a recommendation for a Linux distro when looking for future employment. They would probably know what Linux is commonly used in businesses in Iran, and which would serve you best for training and gaining experience.
If you have the option to buy it, I can recommend the book "UNIX and Linux System Administration Handbook" (fourth edition). This has loads of information for system administrators, and covers topics for all commonly used enterprise Linux and UNIX distros. See http://www.admin.com/
. (It's not a self study guide though, it's a very useful reference for system administrators.)
For LPIC-1 and LPIC-2 you an also pick up a self study guide:
As to your second question, there are probably options but I don't have any experience with that