I work for Twproject, a web based project management system (http://twproject.com) that you can install on your servers.
You can structure your projects having a root for each customer and that a sub-task for every site. Then you can define the sub-sub-task and so on in order to define your project phases and so on.
Consider that yon can attach (drag and drop) several document to each level.
Searchable documents will be indexed full-text; more refined search pages are eventually available.
Every project could be used as “template” for creating new similar ones.
Security is fine grained, and you can give access to project data (you can define it) to your customers.
Additionally it manages issues (can be used as checklist for some tasks), shared agenda, discussions, time tracking, and much more.
Twproject is built on Java and installs everywhere in minutes. It can use your database (MySQL, SQLServer, Oracle, PostreSQL, DB2 etc).
It is easy to customize it adding custom field, custom reports or custom form.
Source codes are available for free under NDA for every customer.
