Confused About Membership Site Options

I am in the process of having a website created. Sales on my site will come from two things:
A store, and a limited time membership area.

The membership area will offer basic courses. There are two levels of courses, and once you've finished them you're done. At that point, if you want to continue learning you would go to the store to buy products that offer more advanced information, or information that was not included in the basic courses.

I thought the company I hired to create the site would be handling the membership part of things along with everything else, but apparently I was wrong. So, I'm now trying to figure out the best way to achieve my goal with the membership portion of the site.

The site is a WP site and will be built on Genesis.

I'm concerned about using a plug-in as I don't want to be responsible for sensitive information possibly leaking out. But, the hosted options are a bit pricey. This is a new business and until it's making some money I need to run it on the cheap.

I will probably have to install or connect whatever program or software I use myself and I'm not technically inclined at all.

Does anyone have any suggestions? I've looked at so many things and I'm completely confused. I want something safe and secure, and easy to use from both the admin side and the log-in side. I also want something that has excellent support - as in if I have a problem I can let someone know and THEY will take care of it. Does such a thing even exist?

Thank you for any help you can give me!