SBS comes by default 5 CALs which can be use either devices CALs or User CALs. There is no need to buy the CALs for the included product such as exchange and SQL because these CALs are bundled for the products included in the SBS 2003 R2 (standard or premium). If a network outgrows to more than 75 users/devices then it can be upgraded using a Transition Pack without loosing your previous invent on SBS2003 R2.
In a case of a bundled hardware from HP/Dell you can have SBS2003 R2 OEM version. A OEM version can’t be transferred to another hardware so it must die with the existing hardware as per Microsoft licensing policy.
On the other way around you can go for an open license for SBS2003 R2 then you will have the option to transfer it from one to another.
Same licensing applies for most of the products.
Always buy the software from registered partners and consultants to make the right investment of software.