And if you ever change providers, then you will need to change your URL as well. If you have your own domain name, then if you ever have to change providers the name goes with you. The good thing about using someone else's domain name is that it is cheaper. You do not need to pay the fees associated with registering your own domain name and you do not need to pay the monthly fees that a web provider will charge you for maintaining your separate domain on his site. By this time your company is receiving so much business from the web that you can afford to pay the start-up costs of putting up your own server and the monthly fees that come from being connected to the Internet.
Cost is the big issue here.
T1 or partial T1 lines coming into your building are expensive - for both the equipment and monthly charges. ISDN is not as bad, but unless you need the high speed access then a typical 28.8k BPS modem on your current phone line should suffice. If this is the case, then you probably don't need something this large right now. The good thing is that there are many local businesses that fall within that range who can offer excellent service. Which company you select will depend on where your business is and how long you want to stay connected to the Internet.