purchasing by monthly installment

The only way that can be done when purchasing at an Apple Store is if you have an Apple Card.

If purchasing at a a third party retailer, you will need to see what options the may have. Some online retailers may also have financing options and "upgrade" programs, i.e. pay so much a month and after a couple years simply turn in the old laptop for a new and continue making monthly payments. These will require credit checks.

Another option is to check out what your credit card company may offer. Some have promotions for you to charge a purchase, such as a laptop, and spread it out in fixed monthly payments.

Go to an Apple Store or contact Apple support to ask this questions, this is an user to user forum so we cannot help you