Little Busters is made up of the main 2 cour series, Refrain and EX, if you have all that you're good - there is also one or two bonus OVAs about Kud that were produced separately via some form of crowdfunding (perhaps Kickstarter but I'm not certain) - having not seen them yet myself, I would be surprised if they would not be enjoyable to watch for someone who likes Little Busters, but AFAIK you don't need to watch them to understand the storyline of the three main parts. FWIW I have seen the first season of Little Busters and thought it was very cute and emotionally moving, but it depends on what you like obviously, I have Refrain and EX too but not watched them yet, I bought the Blu-rays but I did wait for a sale to get them, that doesn't really reflect negatively on the series in this case for me, tbf. Anyway, I hope you greatly enjoy at least one of the series you purchased