Note. OpenCart 1.5.x workrs with VQMOD, OpenCart 2.1 and later - OCMOD. They are quite different. OCMOD is OC native technology. I suggest you to use OpenCart no less then 2.3, better 3.0.3.2.
OCMOD is used, when we need to do changes something in core files, without doing it directly. OCMOD - is a set of instructions, to find a file, tile a certain line(s), replace it or arr code before or after it. Use it when you need to overwrite core files.
Extensions/Modules - mostly something that you going to show on the client side of your store.
Extensions/Tools - have almost the same structure and login, as modules, but you can place it wherever you want in admin menu (using OCMOD). You can use modules or tools, there is no big difference and strict rules.
As I see your process - you will create some your own extension (model, controller and view), and some small part in OCMOD, to correct some core files (if necessary). Don't forget, in OCMOD you can't change .css and .js files. Only .php, .tpl and .twig. That's all I can tell you according to your question.