When deciding what to buy the issue is not as much capacity as it is features, cost and scale. Here is some questions you should ask:
1. What is your customer’s budget?
2. What key business features do they need?
3. What key business features do they want?
4. What are their growth projections?
This will help you pick a decent key system.
Rommel already asks some good questions that you need to ask.
On the hand, you also need to be aware the capacity expansion, if you buy a system and it will reach 80% utilization, then you will face difficult situation soon.
I do agree with alex and rommel. You need to see what is your requirement in terms of how many E1’s and how many extns.(within that digital and analog) with expansion capacity down the line for the next 3 years. ThePBX you choose should be able to cater to this need. Unless you are looking for a very heavy traffic handling capacity i think that you do not have to worry about erlang / switch capacity etc. as almost every pbx comes with a decent traffic handling capability.