Your sales model will be affected by whether you're selling B2B or B2C. B2B will more often bring higher revenues per sale, but require longer sales cycles and more resources to support. Whereas consumers buy higher volumes, faster, but at lower revenues (generally speaking). You will still need to support them, but the people and processes that support them may be slightly different compared to B2B.
Will you sell direct to consumers? If so, do you need to consider the technology you will use to transact; i.e. Apps, e-commerce platforms, payment gateways? Oh, and don't forget about the app store commissions when building out your cost of sales in the financial plan.
If you're making a B2B product or service, who is going to sell it? Are you going to need to create inbound/outbound teams of sales development reps (telesales), external account executives and people focused on customer success? Does your offering require a partner network to reach your desired audience? Therefore do you need to employ folks with channel management experience? And how will you support your partner network? They will need sales, marketing and technical support.
Timing for new hires:
You need to make sure you have the resources ready to grow the business; timing is everything. When do you need to bring people on, in order to give them enough time to ramp up to full productivity? How do you stagger new hires to maintain growth momentum? What triggers are you looking for in order to justify hiring? For example, are they sales revenues, customer numbers, or pipeline size?
If budgets allow, consider hiring two people each time. If you're building a sales team, having two starting together can increase their ramp time as they can share and learn from each other.
Be realistic about ramp times. Until you have a repeatable sales process you have to expect to test different models until you find one that works.
Sales processes will change depending on model, so I am not going to go into any detail in this post as it is not a business plan issue.
I recently read two books on lead generation and sales, by Aaron Ross and Marylou Tyler 'Predictable Revenue' & ‘From Impossible to Inevitable’ also by Aaron Ross and Jason Lemkin. I can't recommend them enough in terms of real world experience, based on Aaron's time at salesforce.com. He provides a lot of useful practical advice that you can implement in your business.
Please let me know if you have any specific sales model questions and I will do my best to answer them.