Well, the good way to emulate a user is Selenium (real browser controlled from the web app). Selenium was developed for acceptance testing purposes, but I'm not sure if it supports proxies.
Another(and cheap) way to do that is using cURL from php, but it does not support Javascript and keeping connections, though it can send requests and process responses, emulate crawling with cookie support. You have an option of multi-threading with dedicated cookie and proxy for each connection. Also, I have a great proxy aggregator written by myself, so you'll get a possibility of 2k proxies online at any time. But we need to discuss this to figure out what option do you really need.
Open for your suggestions and waiting for a PM.