Hi, your question and your statement following are about two different things. A video recording and a webinar are not the same thing.

A webinar is a live event that is recorded into video format. You invite people to attend, get them on a list, remind them to appear at the right time and place, then present live. Afterwards, your webinar is available as a video--though some media type conversion may be required--and you can use it as a "rebroadcasting".

The second thing you described, in which you would be talking over a Powerpoint presentation to no one, is a marketing video.

Yes, the end product is similar; however, the webinar is presented to live prospects.

If you want to record a voiceover on top of slides, and then use this to present an offer at the end, this is called a Video Sales Letter or VSL.

At any rate, you have all the stuff you need to make a marketing video or VSL. You may benefit from buying a microphone or headset with built-in mic if you don't already have one, as laptop microphones are generally pretty bad.

Screen capture your slides and your voice, edit as needed, upload to Youtube, Vimeo or the platform of your choice. After this is done, you'll probably want to embed it into a web page with some sales copy around it and a Buy button.

Then you'll need a way of driving traffic to that page, and probably a squeeze page and autoresponder to capture the email addresses of those subscribers.

If you're going to run a webinar, you could use GoToWebinar, which is the sort of default standard (it's not great, but it has been around forever), another platform or even Google Hangouts On Air. After recording, you'll have the video file--convert from GTW with Handbrake or download from Youtube--to edit or embed as you wish.

So figure out whether you want a live audience or not. If you do, then you'll need a way of attracting and signing up that traffic, and then presenting to them. If not, you can simply record your presentation. But either way, to make use of the recording afterwards you're going to have to have a way of driving traffic to it, and a way for people to buy from you...and that's probably going to be a web page.