Building a 24 workstation network

I do tax and business rules consulting for a mental health practice with a staff of 12 professionals and 5 administratives. They currently occupy 2 offices and have built a new office in which to consolidate their practice. The new office has about 4,000 sq. ft. and a "server closet" located near the center of the office space. The longest wire run would likely not exceed 100' and, on my advice, Cat 5 data cable has been run, originating fron the "server closet", to the 24 data connections in the building. I have been asked to provide an Executive Summary of their needs for the following LAN.

--18 workstations being served DSL Internet
--6 workstations being served DSL Internet and administrative applications for billing and CMS (Client Management Software)
-- 24 workstations being served an Intranet

Cost is a significant consideration for this practice. The mental health profession has not been treated well by the insurance industry and the limits that have been set for reimbursement have not changed for many years. The practice contiues to provide much needed services yet their costs continue to rise but their revenue streams are being controlled by the insurance industry and the government run Medicare system.

I want to offer open-source solutions, as much as is practical, for their consideration. I have maintained the technology infrastructure in the 2 offices for 3 years. But, I have used 4 port routers for the administrative tasks and wireless connections for the professionals. HIPAA (Health Insurance Protability Accountability Act) has a stringent privacy component and I feel that wireless is less secure than I prefer have advised them to provide hard connections rather than wireless in the new facility for security reasons.

The administrative workstations are currently DELLs of various ages with none being more than three years old and running Windows XP Professional (SP2) on three and Windows Vista (sp1) on the other 2. Each professional has their own laptop and currently connects to the network for Internet service; I maintain the laptops on an as needed basis as they are not part of the Practice hardware. There is currently no Intranet but one will be developed.

Their CMS system is vintage 1993 (DOS) and limits their flexibility in data collection and maintenance. I have designed a GUI environment with Microsoft Access but it is not yet implemented in anticipation of the new office environment. In the near future I want to convert the CMS system to a MySQL database and Open-Office office software suite.

My questions deal with the technical issues of delivering Internet and Intranet to the 24 data connections and serving the 6 administrative workstations with their CMS software. I seek the advice of an Expert to determine what hardware I need to handle the LAN I have described. If more information is needed, Please let me know. I am offering maximum points to the best, most practical and low-cost solution to this problem.