I would recommend using atleast 2 physical servers in this scenario. One server would house your Domain Controller, Email Server, File Server and Quickbooks just fine. I recommend something with around 4gigs RAM.
You will want to run your SQL database on its own stand alone server. Depending on how many database queries are present (assuming all client PC's run software that utilize the database) I would recommend a server with at least 2 - 4 gigs of RAM and a quad-core xeon processor. You could easily run a dual-core if you want since the workload would be pretty minimal.