Journal, Funding and Help with Modification

We are a small group of people, and we don’t have funding. We need to raise money for the journals. I am managing the journals, but I am not a programmer. I can make modifications.

Can someone please tell me which file I need to modify to disable lifetime or perpetual download of an article once a buyer pays for it? I want 1 pay = 1 download. Can someone tell me what code has to be entered to achieve that goal?

Can someone please tell me the code needed to offer a yearly subscription? Which file should I modify?
Annual subscription is standard in a lot of academic journals, but there is no option for this in OJS.

When people subscribe, we want to give them access to only the issues they are interested in. Is there a way to disable access to all back issues? Perhaps, the better way to achieve this is to offer a subscription to an issue. Which file should I modify? Can someone please tell me which code is needed to achieve this?

Is it possible to offer 2 different prices for individuals/students, and institutions when buying an article? We want a price for individuals/students, and a price for an institution when purchasing an article. Can someone please tell me which code I need to add to make this change?

Are there any developers or programmers who can offer their services?

We are using OJS 2.4.x, but have not migrated yet to 3. Solutions should be for version 3.1.x