Hey I already learned how to turn my desktop into samba server for my home network. My router and smb config are limited to only local IP's. So I'm pretty happy with that, they are all sharing files fine.

Now I want to put some files available to the public as well for people to download from me 100mb/month per IP and learn how to protect my network as well?

Can samba do this already? or do I need something else?

Samba is designed for LAN's and not for external connections. Maybe look here

You can do the above with iptables but you would need to restart the rules every month since it has no effective restart (besides pulling down the rules and putting them back up)
I believe the quota match is in kilobytes so for 100MB..
Something like:
Assuming the policy is drop, this will accept up to 102400kb (100mb) and then stop accepting till the rule is removed and placed back. If you put the process into a script that runs every month to reset all rules, this could work well.
This post explains quota too

What distro are you running?

Have you checked out VSFTP?

Thanks I'll go with VSFTP and use iptables to set the rules, if needed.

My distro is under my username.