What are the key sales positions (or functions) that a B2B software company should consider filling?

It depends on the type of application you are providing.  If it is a larger consultative sale, such as ERP software, you need to look for people with experience and often a contact base within the area you are servicing.  Once you have those in place you can look to developing a farm team of lesser experienced potential that you you can train and move into larger roles as they evolve and your business grows.

If it s a smaller offering that needs more volume such as a subsrcirpiton service of some sort you may just need energetic phone callers who can get the most opportunities in the pipleine.