There are going to be many different ways you can solve this little problem. I guess it all comes down to exactly what your business needs are (is this a publishing portal, document management system, team collaboration area, will you be need Office Integration etc) and what’s the size of your wallet. What side of the firewall do you want this box to be on? It sounds like your questioning your architecture so are you looking to split it onto separate boxes, not a bad idea if your looking for performance increases and redundancy, but again depends on what you want to use it for.
You are very correct in stating security is going to be a big concern, glad to hear your facing up to it :-). Based on what you've said, single server implementation (I assume its behind your firewall right now on the corporate network). There are many different ways you can solve this issue and if you want to keep that box in the right side of your firewall the 2 that instantly spring to mind are a) by using some sort of remote access gateway which will do most of the hard work for you (as long as its setup correctly) something like a Juniper box:http://www.networkworld.com/news/2004/062804juniper.html or b) by using a reverse proxy to send traffic through your firewall and onto your SharePoint server, either way remember SSL is your friend.
We have both mechanisms setup here and both have their advantages and disadvantages, mostly around costs and functionality. I'll be honest we haven’t done much remote access gateway testing so I cant say for sure which one is best. With regards to the reverse proxy idea we have implemented this but this method requires us to use Forms Based Authentication which presents a whole new challenge and understanding. Either way your going to face possible reduced functionality certainly with things like Office integration.
That’s our experience, sorry to say based on what we've done so far its not going to be an easy one, unless someone else has the perfect/better solution (which I would be very interested in myself) I hope you'll find something useful in this post.