My brother is starting up a small business building and selling computers. We know that selling computers without an O/S can place people off and so are looking to buy OEM software such as operating systems but we aren’t reasonably clear what we need to buy.

For example, if we wanted to install Outlook on our systems to sell, would we only need to hold Windows Outlook OEM or do we need to buy other stuff?
I read somewhere that you also need to buy some licensing kit too, starting to sound expensive.
How many times can an OEM operating system be installed before licenses run out or something?