From personal experience, I can suggest learning Selenium Java bindings to control a browser instance. This will take care of dynamic sites that rely on javascript and ajax, so you will be able to automate actions on virtually any website.
You can use Firefox if you want to actively monitor the bots or PhantomJS if you want the to run them in a headless environment. It is quite simple, and if you have programming experience, you can learn this in 1-2 weeks. Java has good multithreading support so you can write fast bots that run simultaneously. Use Linux as an environment and schedule your bots with cron jobs.
Good luck to you. Cheers!