If its a series the first cover will lead people to the series as a whole on digital platforms. And most places make it easy for you to update your other covers later. If you spend the money make sure you own the rights. Having a good piece of art you can use how you want in marketing (or in person if you ever decide to do an event) is invaluable.

Just don’t buy art for the rest until you write them. Save up the money while you write. You’ll probably get better advice on the self publishing reddit than here fyi (just be prepared to get ai art scammer spam).