Build a web application load test

We have a multi-tier web application. It has been falling down under heavier user loads. We need a test suite to run and generate simulated load so that we can troubleshoot the problem without impacting normal users. Our need is immediate, and funds are available. Our application consists of a web site front end that talks via web service calls to a Learning Management System (LMS). Users log into the web site and are presented with a list of courses assigned to them on the LMS. They click on a course from this list. The web site communicates with the LMS to launch that HTML course in a separate popup window. This new course window has "Next" and "Previous" buttons to navigate through the course. There is a second user interface that allows the user to bypass the web site front end and log directly into the LMS. The web page layout is different in this interface, but the result is the same; they are presented with a list of assigned courses with links to launch each course in a new popup window (the same popup window that gets launched through the web site front end). Both the web site front end and the LMS back end are hosted on Amazon Web Services (AWS) EC2 instances. I have spent hours on the phone with AWS support. They have been unable to find any configuration options to explain our observed problems, which are high latency and server (500) errors; however, the load was not sufficient to trigger the problem while they were looking at it. I need code to simulate multiple users so that we can cause the problem when we are prepared to further troubleshoot it. We are not even sure if the problem is in the web site front end, the LMS back end, or the communication between the two. Therefore, I need two test suites: one that uses the web site front end , and a second that talks directly to the LMS back end. I need to launch each test suite with a parameter to set the number of simulated users for the current run. We will configure a group of test users for the test suite to use - each test instance must log in as a separate user. I have been looking at Selenium for this, but have recently read posts that say this is not the best tool for load testing. I am open to suggestions for alternate test frameworks. Whatever code is generated to perform these tests will delivered to and become the property of Brindle Waye, LTD. Source for all executable modules must be a part of the delivery.