Can I buy this app for a third-party? Can I buy this program as a private person for my business?

Yes. Anyone can pay for/ buy this program for a third person or business.

Is there a limit on the number of seats per licence?

A licence for SCS is required for each computer on which the program is installed, regardless of the number of users working with the program on this computer. One computer – one licence.

Is MS Outlook required to send mail from this program??

No. No third app is required to send email out of SCS.