There are many factors to consider when choosing a domain name for your new website. Most people would choose to go with their company name, or if they can not get it, some substitute of it. For example, if your company was Green Bottles and www.greenbottles.co.za was already taken, then you could look at www.green-bottles.co.za; or www.greenbottlesco.co.za; or www.sagreenbottles.co.za. The other alternative is to go for a domain name that describes what you do, so if you are a company that prints business cards, you could go for www.businesscards.co.za; or www.weprintbusinesscards.co.za; or www.businesscardprinters.co.za etc. The options really are endless, as long as the domain name has not already been registered by someone else.
No, should you go for a co.za or a .com? For me this is a very simply answer. If your company is based in SA, serving SA customers, then always go for the co.za. However, if a large (more important) part of your customer base is situated outside of South Africa, then yo should go for the .com.
If the domain you want is currently taken by someone else, and you really want it, then there is nothing stopping you from contacting them directly and seeing if they will accept an offer on it. Purchasing a domain name from someone is just like purchasing anything else, in that the price is based on what you are prepared to pay and what they are prepared to accept..nothing more.