Well, some of the ISP's that service residential areas do not allow hosting web sites, so I would say that this should be one major point that should be taken up with them before you start in on the project. Most of the time, there are alternate service providers you can go with, but definitely something that you will need to know before hand. Also, another big point to cover will be the upstream offered from your ISP. Most ISP's will tell you that they will give you 1.5 mbps (or better) down stream, but on the upside, you would be lucky to get 50 - 100k out of it. This is fine for residential customers surfing around on the web, but for a server, and especially a business oriented server, it will be a killer. May want to get a good understanding of what their downtime policy is as well. In most residential accounts, if there is downtime on the lines, you don’t get anything for that. Its just part of the low prices you are paying. But for your type of installation, you will need some kind of service standards in place so that you don’t find your server down for 4-6 hours, customers can’t access it, and the ISP can’t or won’t do anything for you. There are many other things to consider, but these are just a couple of things that you should have an understanding of before you start.
Hope any of that helps!
-Jason