Yes, you do need to obtain a visa to work in Al-Khor.
It depends on your job position. Generally, it's advantageous to know some of the language, but not necessarily mandatory.
Typically, a relevant degree in sales or a related field is required. In addition, some experience in the industry may be beneficial.
Yes, it is possible to open a business as a salesman in Al-Khor. Depending on local laws and regulations, you may need to obtain a trade license in order to legally operate a business.
It is advisable to purchase health insurance prior to taking up employment in Al-Khor. Additionally, some visas will require you to obtain health insurance from local providers.