I am considering buying another domain for my site. Possibly wanting to buy .sg domains. Is it possible to buy when my company is not in Singapore, and I am not Singaporean?
Sure it's possible.
Using a country code top-level domain could have negative implications for your website's SEO but you could always use a different domain as your primary domain and forward your .sg domain.
It shouldn't hurt your SEO to simply have another domain (unless it has spammy backlinks) but make sure to set it as a secondary domain and redirect traffic from the .sg to your primary domain.
I'm also curious why you want a .sg domain (I can't think of a domain hack that would end in sg).