Hello,
I'm Vladimir, i'm working for 5 years as a PHP developer on a local payment processor so i can write secure code with no vulnerabilities.
I maintained 2 opencart shops so i'm used to this platform and i can create a module as you requested for the membership functionality. It will require vqmod to be installed on your opencart instance, if you don't have it already i can help with that too.
The development will take place on my own hosted instance of opencart if you agree i can copy the theme on my own server. It will take me 2 days in total, including the testing and fine tuning it after your feedback.
LE: I already have 85% done of the module, it just need to be customized for your theme as you want it to be and to apply the price discount logic (now it use the price discount as the new price (not substraction))
I added an option to edit the membership expiration date directly from the customer edit menu, and an option to set the number of months a product will give to customer if purchased.
Some screenshots: [login to view URL]