Its up to you what type of server you go with. If you go name brand you usually end up getting a support package with the server (next day replacement or something of the sort if you pay for the nice packages) which can make the difference. If you go with a whitebox (custom) solution you usually are responsible for taking the part that goes bad, sending it to the company, they then send you the replacement, and it can take weeks sometimes.
As mentioned above there really is no way for us to give you the specs of the server that you will need. It depends on the actual number of users per day, hour, minute, and at the same time as well as the needs of the application itself and in addition the bandwidth requirements, etc...
The machines you have listed above would be a good start, and it seems like you would be able to scale your application with very little trouble so you should be just fine to expand as you need to.