I am just curious as to how to set up a website - I can do simple web stuff like html, css, etc.., but is it better to go to one of the sites that have pre-configured webpages that I can choose the style appropriate for me. Might this be faster but would it cost me time trying to edit it all the time?

But my big question is, if I use one of those sites, can I download the web-pages to my own machine to work on it, or do they only allow this for a fee?

Right now I seem to be going pretty well just starting from scratch - I can load a page and navigate pretty well using css type stuff.

Just curious if I'm reinventing the wheel too much but there are many ways to skin a cat in web pages so it doesn't seem all that hard - until I realize that I will be running under XAMMP against MYSQL and I will have to make databases, and do a lot of TCP/IP stuff for all of it to work. Then it might get time-consuming - like ordering stuff, inventory, ads, etc..

But my big question is, if I use one of those sites, can I download the web-pages to my own machine to work on it, or do they only allow this for a fee?

Difficult to say, it depends on the site and their conditions.

Since you know HTML and CSS I recommend you stick with it so you have full control over it.
To make things a bit easier you can get a template so you can easily modify the design and layout.
Some free templates require back links but if you don't mind to spend a couple of dollars you can have a look at https://codecanyon.net/popular_item/by_category?category=html5