In your case I would recommend a one month long subscription. If you subscribe for one month you can download everything you want and try out all Soundpools until you find something you like. You can use the Song Maker to create ideas very fast.
Cancel the subscription via Music Maker and buy the commercial license for every Soundpool you want to use for your movie if you want to sell it or place ads on it. If you just want to show it for familiy and friends this is not necessary.