Personally I use a combination of Dreamweaver, Photoshop and webassist dreamweaver extensions for the content management. These days you need to be looking at responsive designs so the site looks good no matter what device you are viewing it on. Dreamweaver cs 5.5 onwards supports responsive designs and also supports the jquery mobile framework. There loads of free php forums you can use like mybb and phpbb. If I were you I would try and learn the basics of html5 and css3. Some knowledge of PHP AND SQL is helpful but not essential. Likewise with jquery and ajax.
Databridge 3 which are dreamweaver extensions includes everything you need to create a dynamic website. The cost of that is $399
You obviously need dreamweaver as well
Of course you can use wordpress but I have always designed my own sites and built my own content management so cant comment on that.
To build a site for what you want and make it secure would be pretty easy really, depending what payment gateway you will use. Paypal express is easy to incorporate. With paypal express you don't need to worry about ssl certificates which can be expensive, also no monthly fees with paypal express. The problem maybe the bandwidth if people are downloading and streaming videos, so that's where the extra costs may come in.