Time and Money – it is always about time and money.
If you have the time for the learning curve then use L_A_M_P [Linux, Apache, MySQL, Perl/PHP] lowest cost overall but the most time consuming it you are not already a Linux user. Easily handle many more users, fast web services, thousands of frontend programs to provide the glue between database and web.
If you are a ‘Windows’ shop and like it that way. Setup a W2k3 server, IIS 6.0, SQL Server 2000 and if it fits ‘SharePoint Services’ [MS Portal]. You can use many methods to control the ASP.NET feed between web and database. (Personally i like DreamWeaver.)
Somebody is sure to recommend MS Access (a good program), BUT not suitable for your projected database size, and you did not say if MS Office is installed on all your clients.
Last are the dedicated solutions with internal web servers and fully capable of locking you into a solution. Zope is a perfect example. If you know exactly what you want and can write detailed specifications, then a programmer can deliver a complete package at a reasonable cost. DON’T plan on any changes though. Changes can cost as much as the original progam. [Basically changes are a complete new program.]
Good luck and welcome the wonderful world of web services. Just make sure you are ‘the man behind the curtain’ and know how it works