Hiya,
Heres a few considerations:
1: In regards to AV there are a few solutions:
a: Use SharePoint integrated AV only. This will scan your SP files and SP files only. No server scanning.
b: Use Server scanning only. This will scan only files on your server and not files within SharePoint.
c: Use both.
d: Use none.
Which one you need depends on your usage pattern and users. If you already have AV scanning on your client computers are they are the only ones uploading, you should not need a or c. If you only access your servers from protected computers, you should not
need b. IF you do both, there would be no problem in choosing d.
When you want to expose a solution to the internet, there are quite a few things to consider if we take everything into consideration. The network placement of your front end servers should be DMZ, with only specified ports open to and from App server and
database server. App server and database server could be placed within your normal LAN. That way your minimizing your attack surface as well as minimizing the routes available if second line of defense is broken.
Now as for SharePoint, you need a FIS license, which is quite a different price tag than server + CAL licensing.
In terms of making the solution available you need to create a route from a client to your site. That means. DNS record -> Public IP -> Public IP -> Your SharePoint server -> SP site Name(Your web server should be responding on this name)
I presume that your site is internet type of site, meaning your allowing anonymous and not Extranet type of site, in that case your site also needs to have anonymous access enabled.
If you can provide more details about your solution, it would be possible to be more specific in terms of defining the best architecture :)