Do you really want to/need to develop it for yourself? Haven't you considered installing a closed or open-source implementation, like the ones here (
Top 9 Open Source CRM Systems), or others you can fint with google?
If you can't use any of these, you could however get inspiration, so I suggest you try them.
PS: there are two different approaches here:
1) several on-premise installations with their own database; and with the databases synchronized over network
2) one single installation that is exposed to the internet or over vpn
The approach to have different installations accessing the same database is not a good one.
I am a big PHP fan, but I suggest you consider ASP.NET MVC with C# too (see these for example:
http://sourceforge.net/projects/basiccrm/[
^],
http://crm.codeplex.com/[
^]). But you could use Java also (look here:
http://java-source.net/open-source/erp-crm[
^]).
The question is not stupid, but the topic is too large, and the specification you gave is not to detailed. A CRM can be really complex, so you will have to define (for yourself) a detailed specification, and choose between the platforms based on that and the technical and financial environment you have to align into.
My opinion:
I would choose asp.net mvc in our enterprise environment - even if there are many java and also php applications on place-, because we have a Windows based infrastructure and .net gives me a lot of tools to access and use those infrastructure elements, and because asp.net mvc is a framework that leads yo towards a sustainable, maintainable and cost-effective, standard-oriented web based development.
I would never consider a thick client for such a task. If you are interested, I can explain why.