Robert Sebastien has posted some useful information. If such an option is chosen the LAN needs careful configuring to avoid unneccessary traffic. In this scenarion a virtual web server is the best bet. ISDN is a great option if the ISP can give your flat rate ISDN as opposed to metered and preferable to low bandwidth leased line in that case.
Flat rate gets data fees abover a certain level.
If you mean to go with leased line then for an average office up to 50 people can share 64K just fine unless they spend lots of time web surfing which of course they shouldn't. If you have a team of web developers then you need at least 64K per 8 people and 4 is better. You can run mail on the same host ok. It is very easy to upgrade 64K to 128K leased line but from 256K up the technology and costs are different. If you are lucky enough to be able to get cable or ADSL then that is the option to go for.