Managing complex interactions with registering users on a non-blog website

Hello, WP fans!
This is my first message in the WP forum and I’m not sure whether this is the right section for my questions, but I’ll try.
I’ve been using WP for a week now, playing with it and trying, and I like it for simplicity and elegance. But here I have a serious problem.

I am trying to create a website based on WP where I am planning to sell my shareware. I’m very new to web-programming in general, don’t know even basics regarding PHP and MySQL, but I hope WP will help me on this road. What I’m struggling with is choosing the right plugin for my strategy. Let me explain.

First, I want my users to be able to buy my application through the website. They download it for free, try it, then access the web-site through a given feature within that app and get access to the payment processor on the website. The only way to access the payment processor is through my app ( i.e. you cannot buy it without downloading it first ). The server, therefore, accepts some crucial piece of information while being accessed from the app (but user has no idea what it is; it is actually some string depending on the computer configuration, let’s call it special string). It REMEMBERS that piece of information and, based on that, generates a serial number and sends it to the user’s e-mail once the payment is complete. After that, the user is saved in the database, with additional fields such as the date of purchase, special string etc., for future upgrades and so on.
Another thing I want to add is to let a beta-tester leave a feedback about the app (the same way, accessing the web-site from inside the app, registering and completing a feedback form). Once the feedback is made, the user is saved in the database and the serial is send for free, as a reward for testing.

Fuf, all in all, I’m planning something like that… So my question is… How can I implelent this using WordPress ? Is there a plugin for that kind of a thing ? If not, will it be worth investing time into learning PHP+MySQL from scratch in order for me to write my own plugin ? I’m just afraid that whatever code I’ll write by myself, it might be easily hacked since I’m a novice, that’s why I want WP to take care of the security side of the problem.
I can modify my strategy a little bit if there’s a plugin doing a resembling thing. For example, to avoid communication between the app and the server, I may ask user to enter the special string in a given field on the web-site.
So, as you see, the variety of choices is great, and that’s what confuses me strategically.
Also, I have a feeling I’m reinventing the wheel, since this seems like something a lot of commerical websites do. But the search for specific answers or clues on how they do it has not succeeded so far. That’s why I thought I could ask live people here.

I’ll be glad to hear all kinds of answers! Please offer your solutions and criticize my approach if you think it deserves it. Thanks a lot !

P.S. If you want, I can move this topic to another section (WP-advanced maybe ?)