I'd like to take some time to think about the best thing to do.
I need to think slowly and carefully about this because it's a big decision.
1) I'd like to take some time to think about the best thing to do.
「最善のことが何かをじっくり考えたい。」
take some time to think about で「〜について時間をかけて考える」＝「じっくり考える」という言い方。
2) I need to think slowly and carefully about this because it's a big decision.
「大きな決断なのでじっくり考える必要がある。」
think slowly and carefully で「ゆっくり慎重に考える」＝「じっくり考える」という言い方。
ご参考になれば幸いです！
Think over ＝「じっくり考える」
Make a decision ＝「決断を下す」
crucial＝「きわめて重要な」
Issue＝「問題」
I want to think this over before making a decision since this is a crucial issue.
これは極めて重要な問題なので、決断を下す前にじっくり考えたい。
ご参考までに
日本 
