It seems that this coming vehicle inspection will be expensive because they will probably have to replace a lot of parts.
It seems that this coming vehicle inspection will be expensive because they will probably have to replace a lot of parts since the car is old.
ご質問ありがとうございます。
「今回の車検代は部品の交換が多くなりそうなので高くなりそう」は英訳すれば、「It seems that this upcoming vehicle inspection will be expensive because they will probably have to replace a lot of parts.」になります。
まず、「It seems that this upcoming vehicle inspection will be expensive」の部分は「今回の車検は高くなりそう」という意味です。
そして、理由をあげる「because they will probably need to replace a lot of parts.」は「部品の交換が多くなりそうから」という意味です。
また、「車が古いので」という情報も入れたいなら、最後に「since the car is old.」
最終的な英文は「It seems that this coming vehicle inspection will be expensive because they will probably have to replace a lot of parts since the car is old.」
ご参考になれば幸いです。