Our system has no way of handling recurring payments such as a periodic subscription or installment plan. You may want to investigate just using PayPal's own Recurring Payments solution, either as-is or combined with a purpose-built membership management system such as FrontDeskApp and/or a specialized mailing list service such as Aweber.
The main problem with supporting such a feature is that, even if we could send orders to checkout that would initiate a recurring payment plan, we have absolutely nothing built into our e-commerce system that could provide buyers and sellers with anything of value for those periodic payments, such as tracking membership/subscriber status, administering access to a members-only system, scheduling mailings and/or downloads, etc.