Can i pay for a 50 GB plan all at once, or does it have to be monthly?

iCloud Storage can only be purchased as a monthly subscription

iCloud storage does not add storage onto your device. It adds storage into Apple's cloud storage system, here you can save backups and photos. Just to confirm you are aware of this right?

To view and manage your iCloud storage go here

This means for every month you continue to stay subscribed, you will continue to receive the storage.

For example, you could save space on your phone by keeping low quality/resolution photos on your phone but keep the high-quality originals in iCloud.

Take a look at this help article for more info Set up and use iCloud Photos – Apple Support

If you would rather not have to worry about using a debit card or if you have a payment method that you have to add funds to and you would rather not deal with that, you can use Apple Gift cards to pay for your iCloud Storage. I never use my card for iCloud Storage or for other purchases. I redeem gift cards and always keep a sufficient balance to cover the monthly storage cost and for my other App Store and iTunes purchases.