You might start by considering the following:
1. How many hosts and servers do you need to be connected to the network at the branch office?
2. What do you expect the maximum number of hosts to grow to in 1-3 years?
3. How big is your branch office in terms of floors, office space, number of users?
4. Will you use VLANs to separate business functions, physical locations, etc?
5. In the 3-tier design, are Core and Distribution layers at HQ? If not, what tier will be at HQ?
6. What medium do you expect to use to connect to HQ?
7. What medium connection will HQ support?
8. How much money do you want to spend?
This should be a good start.
